A plurality of field effect transistors are arranged in a line and have isolated open gates respectively. The gates are opposed to a surface of a charge storage medium and are subjected to respective induced voltages corresponding to a distribution of an electric surface potential of the medium. The transistors are enabled to output signals corresponding to the induced voltages respectively. The signals are sequentially transmitted from the transistors to an output terminal to generate an electric signal representative of the distribution of the electric surface potential of the medium.