Thickness detecting sensor

A thickness detecting sensor can widely set a dynamic range for detecting the thickness of a medium, and exactly detect the thickness of the medium even when the dynamic range is widely set in this way. Therefore, the thickness detecting sensor has a fixing electrode (3) covered with resist (34) and a movable electrode (4) arranged so as to be opposed to each other, a plunger (20) for changing the gap between the electrodes by coming in contact with the medium, and a CR oscillating circuit and an frequency voltage converting circuit for converting electrostatic capacity stored between the electrodes into an electric signal for detecting the thickness of the medium. The rear end portion (21) of the plunger comes in contact with the electrode face of the gap side of the movable electrode. The movable electrode is displaced in the direction separated from the fixing electrode correspondingly to the thickness of the medium.