An anomaly in a high-pressure fuel system of an internal combustion engine is diagnosed early on and with high precision, and, further, the anomaly site is identified. With respect to a diagnostic device for a direct injection internal combustion engine, there is provided: an injection correction amount computation means (302) that computes an injection correction amount so as to bring a detected air-fuel ratio to a target air-fuel ratio; a fuel injection valve control means (202) that controls the fuel injection valves with the fuel injection amount corrected based on the injection correction amount; a discharge correction amount computation means (305) that computes a discharge correction amount so as to bring a detected fuel pressure to a target fuel pressure; a fuel pump control means (203) that controls the fuel pump with the discharge amount corrected based on the discharge correction amount; a fuel pressure value shifting means (204) that shifts the value of the detected fuel pressure if the injection correction amount deviates from a predetermined range and until the injection correction amount converges to a given amount within the predetermined range; and an anomaly determination means (306) that determines which of the fuel pump, the fuel injection valves, and the fuel pressure sensor has an anomaly based on the discharge correction amounts before the fuel pressure values shift starts and after the fuel pressure value shift ends, and on the injection correction amount before the fuel pressure value shift starts.