PURPOSE: To measure the dielectric constant of a photosensitive layer non- destructively without contacting by means of the surface potential before and after corona charging, the current flowing in ground potential, etc.

CONSTITUTION: The surface potentials V2, V1 before and after charging of a photoreceptor 4 at a peripheral speed U by a corona charger 6 are read with probes 9, 11 and electrometers 10, 12; at the same time, the average value If obtained by dividing the current flowing in ground potential by the width L in the axial direction of the photoreceptor 4 while the photoreceptor 1 makes one turn is read with a DC ammeter 15. If α=If/(V2-V1) is calculated from these read values, dielectric constant =L×α/(0×U) is determined, and the dielectric constant of the photoreceptor is measured non-destructively without contacting. (0 is the vacuum dielectric constant of the photoreceptor.)