To provide an electrostatic capacity detecting device which is small- sized, thin, and lightweight and can be utilized as a fingerprint image input device requiring low power consumption.

Bias charges are injected into a detecting electrode 11 forming an electrostatic capacity element between it and a subject from a bias power supply line 3 to accumulate charges corresponding to an electrostatic capacity, and the charges are distributed between the detecting electrode 11 and a capacitive element 12 reset to a reset potential. The holding voltage of the capacitive element 12 is changed in response to the charge quantity accumulated on the detecting electrode 11, and a signal is outputted as the change quantity of the gate potential of a source follower amplifying element 13, thereby the electrostatic capacity formed by the subject and the detecting electrode 11 can be detected.