A printer determines, in a process control, a reference exposure amount. A charging unit charges a surface of a photosensitive element to a target potential. After an exposure target area on the charged surface of the photosensitive element is exposed to a high adjustment exposure that corresponds to a high-exposure amount area, a control unit determines the reference exposure amount based on the target potential and a detected residual potential that is a result detected by a potential detecting unit as a potential of the target exposure area on the photosensitive element after being exposed to the high adjustment exposure.