ROTATIONAL FREQUENCY DETECTION DEVICE

A rotational frequency detection device equipped with: a blade detection sensor (12), which is provided in the housing (8) of an intake air compressor (6) of a turbocharger that pressurizes intake airand supplies this air to an engine (1) used for vehicle travel, and the output voltage of which fluctuates higher and lower due to the approach and retreat of the blades (10) of a compressor wheel (10) rotating inside the housing; and a sensor circuit (13), which binarizes an output signal from the blade detection sensor into a high signal and a low signal, and outputs the signals. The sensor circuit is equipped with: a low-pass filter (21) that cuts a high-frequency component of the output signals from the blade detection sensor by means of an analog filter or a digital filter; a high-pass filter (23) that cuts a low-frequency component of the output signals from the blade detection sensor by means of a digital filter; and a control unit (28) that raises the cutoff frequency of the high-pass filter when the rota