An injector failure diagnostic device 1 diagnoses injector failures for a multi-cylinder internal combustion engine 2 having injectors 21 to 24, each injector injects fuel to the corresponding cylinder. The injector failure diagnostic device 1 includes: an operating sound obtainment unit 12 configured to obtain current operating sound that is operating sound when all of the injectors operate, and obtain pseudo-failure operating sound sequentially for each of the injectors, the pseudo-failure sound being operating sound generated by stopping the fuel injection from one injector while maintaining the operation of the remaining injectors; and a faulty injector identification unit 13 configured to determine whether or not the pseudo-failure operating sound of each injector obtained by the operating sound obtainment unit 12 is similar to the current operating sound to identify a faulty injector.