Abnormality detection device for automatic transmission of invention has automatic transmission 5A connected to vehicle driving source 1, 3 through clutch 4a; range detector detecting selection range of selector lever 9 and outputting detection signal; manual valve 90 configured to, when selector lever is positioned at drive range position, engage clutch and bring automatic transmission into power transmission state, and when selector lever is positioned at range position except drive range position, disengage clutch and bring automatic transmission into power non-transmission state. Device further has engagement control unit 10B controlling clutch from disengagement state toward engagement direction when no-signal state in which signal is not outputted from range detector continues for first predetermined time or more; engagement judgment unit 10C judging engagement of clutch during engagement control; and abnormality determination unit 10D determining, when engagement is judged, that range detector is in no-signal abnormality indicating that no signal is outputted.