PURPOSE: To accurately measure the amount of offset, which is required for measuring the surface potential of a photosensitive body by making a distance from a potential measuring means to a ground plate, which is positioned between the photosensitive body and the potential measuring means when the surface potential of the photosensitive body is not measured, equivalent to a distance from the surface of the photosensitive body to the ground plate, thereby setting the same positional conditions as when the surface potential of the photosensitive body is measured.

CONSTITUTION: When the potential measuring means 50 measures the surface potential of the photosensitive body 12, the end part of the ground plate 54 is made to move to a position shown by a symbol A, and consequently the detection surface 53 of the potential measuring means 50 is positioned so that it faces the surface of the photosensitive body 12. When the potential measuring means 50 does not measure the surface potential of the photosensitive body 12, the end part of the plate 54 is made to move to a position shown by a symbol C, and consequently it is set in the position where it shields the detection surface 53 of the potential measuring means 50 against the surface of the photosensitive body 12. Even when the potential measuring means 50 does not measure the surface potential of the photosensitive body 12, the means 50 is put in the same positional condition as when the detection surface faces the surface of the photosensitive body 12; therefore, potential corresponding to the amount of offset of sticking substance, such as toner sticking on the detection surface, can be measured in this condition.