diff --git a/atcoder-problems-backend/src/crawler_utils.rs b/atcoder-problems-backend/src/crawler_utils.rs
index 84c8cc4ca..8eea42497 100644
--- a/atcoder-problems-backend/src/crawler_utils.rs
+++ b/atcoder-problems-backend/src/crawler_utils.rs
@@ -244,29 +244,29 @@ async fn upsert_problems(
     new_problems: Vec<Problem>,
 ) -> Result<usize, DbErr> {
     for problem in &new_problems {
-        // Insert into problems table
-        let title = problem.title();
-        let model = sql_entities::problems::ActiveModel {
-            id: Set(problem.id.clone()),
-            contest_id: Set(problem.contest_id.clone()),
-            problem_index: Set(problem.problem_index.clone()),
-            name: Set(problem.name.clone()),
-            title: Set(title),
-        };
-        sql_entities::problems::Entity::insert(model)
-            .on_conflict(
-                OnConflict::column(sql_entities::problems::Column::Id)
-                    .update_columns([
-                        sql_entities::problems::Column::ContestId,
-                        sql_entities::problems::Column::ProblemIndex,
-                        sql_entities::problems::Column::Name,
-                        sql_entities::problems::Column::Title,
-                    ])
-                    .to_owned(),
-            )
-            .exec(db)
+        // Check if problem already exists; only insert if it doesn't.
+        // This prevents overwriting existing problem metadata (e.g. original contest_id)
+        // when the same problem_id appears in multiple contests (ADT reuses ABC problems).
+        // In other words: this is an ADT-specific countermeasure to keep the first-seen
+        // contest as the canonical one for the problem.
+        let existing = sql_entities::problems::Entity::find_by_id(problem.id.clone())
+            .one(db)
             .await?;
 
+        if existing.is_none() {
+            let title = problem.title();
+            let model = sql_entities::problems::ActiveModel {
+                id: Set(problem.id.clone()),
+                contest_id: Set(problem.contest_id.clone()),
+                problem_index: Set(problem.problem_index.clone()),
+                name: Set(problem.name.clone()),
+                title: Set(title),
+            };
+            sql_entities::problems::Entity::insert(model)
+                .exec(db)
+                .await?;
+        }
+
         // Insert into contest_problem table
         let contest_problem = sql_entities::contest_problem::ActiveModel {
             contest_id: Set(problem.contest_id.clone()),
diff --git a/atcoder-problems-backend/tests/test_crawler_utils.rs b/atcoder-problems-backend/tests/test_crawler_utils.rs
index 045f593be..eb6693d13 100644
--- a/atcoder-problems-backend/tests/test_crawler_utils.rs
+++ b/atcoder-problems-backend/tests/test_crawler_utils.rs
@@ -402,3 +402,93 @@ async fn test_crawl_contests_fetches_filtered_archive_categories() {
             .any(|contest| contest.id == "adt_all_20260612_2")
     );
 }
+
+#[tokio::test]
+async fn test_crawl_adt_problems() {
+    let db = setup_db().await.unwrap();
+
+    // 1. ABCのコンテスト、問題がクロールされる
+    sql_entities::contests::Entity::insert(sql_entities::contests::ActiveModel {
+        id: Set("abc001".to_string()),
+        start_epoch_second: Set(0),
+        duration_second: Set(0),
+        title: Set("AtCoder Beginner Contest 001".to_string()),
+        rate_change: Set("?".to_string()),
+    })
+    .exec(&db)
+    .await
+    .unwrap();
+
+    let mut abc_fetcher = MockProblemFetcher::new();
+    abc_fetcher
+        .expect_fetch_problems()
+        .withf(|contest_id| contest_id == "abc001")
+        .times(1)
+        .returning(|_| {
+            Ok(vec![Problem {
+                id: "abc001_a".to_string(),
+                contest_id: "abc001".to_string(),
+                problem_index: "A".to_string(),
+                name: "Problem A".to_string(),
+            }])
+        });
+
+    atcoder_problems_backend::crawler_utils::crawl_problems(&abc_fetcher, &db)
+        .await
+        .unwrap();
+
+    // 2. 次に、ADTのコンテスト、問題がクロールされる
+    sql_entities::contests::Entity::insert(sql_entities::contests::ActiveModel {
+        id: Set("adt_all_20260612".to_string()),
+        start_epoch_second: Set(0),
+        duration_second: Set(0),
+        title: Set("AtCoder Daily Training 2026/06/12 All".to_string()),
+        rate_change: Set("-".to_string()),
+    })
+    .exec(&db)
+    .await
+    .unwrap();
+
+    let mut adt_fetcher = MockProblemFetcher::new();
+    adt_fetcher
+        .expect_fetch_problems()
+        .withf(|contest_id| contest_id == "adt_all_20260612")
+        .times(1)
+        .returning(|_| {
+            Ok(vec![Problem {
+                id: "abc001_a".to_string(),
+                contest_id: "adt_all_20260612".to_string(),
+                problem_index: "C".to_string(),
+                name: "Problem C".to_string(),
+            }])
+        });
+
+    atcoder_problems_backend::crawler_utils::crawl_problems(&adt_fetcher, &db)
+        .await
+        .unwrap();
+
+    // 3. abc001_aがabc001とadt_all_20260612の両方に紐づいていることを確認する
+    let problems_after_adt = sql_entities::problems::Entity::find()
+        .all(&db)
+        .await
+        .unwrap();
+    assert_eq!(problems_after_adt.len(), 1);
+    assert_eq!(problems_after_adt[0].id, "abc001_a");
+    assert_eq!(problems_after_adt[0].contest_id, "abc001");
+
+    let contest_problems = sql_entities::contest_problem::Entity::find()
+        .all(&db)
+        .await
+        .unwrap();
+    assert_eq!(contest_problems.len(), 2);
+    assert!(contest_problems.iter().any(|cp| cp.contest_id == "abc001"
+        && cp.problem_id == "abc001_a"
+        && cp.problem_index == "A"));
+    assert!(
+        contest_problems
+            .iter()
+            .any(|cp| cp.contest_id == "adt_all_20260612"
+                && cp.problem_id == "abc001_a"
+                && cp.problem_index == "C")
+    );
+}