In order to compensate fluctuation of the characteristics of a field effect transistor and detect capacitive change of a capacitive element for measurement with high precision, the electronic circuit of the present invention includes; a transistor outputting current in response to voltage change, which is supplied from a variable voltage source to a current control terminal by capacitive coupling via the capacitive element for measurement; and the constant current source to output reference current; and the current detection circuit to detect output current outputted from the transistor. After making the reference current pass through the transistor and compensating fluctuation of threshold voltage, a switching element is turned "OFF" so as to make the current path between the gate and the drain of the transistor to be non conductive. The voltage applied to the gate electrode from the variable voltage source via the capacitive element for measurement is changed thereby and output current level is detected by the current detection circuit so as to obtain the changed amount of the capacity of the capacitive element for measuring.