Provided is a diagnostic apparatus, including a diagnostic unit configured to diagnose a condition of a vortex flowmeter using a determination result of a magnitude of a signal component of each of at least one target detection signal among the at least one detection signal detected by the vortex flowmeter having a vortex generator and a detection unit configured to detect at least one detection signal according to a vortex generated by the vortex generator, or a magnitude of a signal component of at least one of combined signals configured to linearly combine two or more of the at least one detection signal.