PURPOSE: To provide a device for measuring the characteristic of a photoreceptor drum capable of accurately measuring exposure without previously measuring light quantity at a position equivalent to an exposed surface or applying the measured value of an illuminance measuring sensor by putting the sensor around the exposed surface.

CONSTITUTION: The characteristic measuring device measuring electric characteristic by performing at least charging and exposure to the photoreceptor drum is constituted of a charging means 2 for charging the photoreceptor drum 1, an exposure means 3, a half mirror 4 branching part of exposing light in the case of changing the exposure and measuring potential damping amount, a light quantity sensor 5 measuring the quantity of branched light, and a potential measuring means 6. A light source, the half mirror and the light quantity sensor can be set outside an environment chamber in which the photoreceptor drum is housed.