PURPOSE: To make a processing circuit common and versatile for processing various kinds of electric signals, such as voltage, current, power, etc., outputted from a sensor head and reduce the cost of a physical quantity detector.

CONSTITUTION: The electric signal of a sensor head 2 is converted into a voltage signal by means of a voltage conversion circuit section 3. The voltage signal is impressed upon the small-gap capacitor 8 of an oscillation circuit section 4 and converted into a frequency signal by means of the section 4. The capacitance C of the capacitor 8 and the oscillation frequency (f) of the frequency signal vary in accordance with the voltage signal. A detection circuit section 5 detects the oscillation frequency of the section 4 and outputs the detected frequency after converting the frequency into a signal indicating physical quantity information.