A voltage is electrostatically induced at a sensing device in correspondence with a surface potential of a measured object. In the case where the sensing device uses a field effect transistor in transferring detected information, there are some factors decreasing the accuracy of detection. Various new arrangements prevent such decreases in the accuracy of detection. In the case of the surface potential of the measured object corresponds to an optical image, new arrangements ensure accurate detection of the optical image. In some cases, color component signals are obtained on the basis of a detection signal.