An inertial detection circuit includes a signal generator and a processing unit. The signal generator includes a drive voltage generator and a test voltage generator. The drive voltage generator applies to the fixed electrode a drive voltage with a first frequency. The test voltage generator applies to the fixed electrode a test voltage with a second frequency lower than the first frequency. The processing unit includes a detection processor and a diagnostic processor. The detection processor performs a detection processing of detecting a capacitance between the fixed electrode and the moving electrode based on a detection signal. The diagnostic processor performs a diagnostic processing of diagnosing abnormality of the detector based on a diagnostic signal.