A process unit includes a first shaft portion at a first end of a photoreceptor drum that is fitted into a hole of a first receiving portion so as to move the photoreceptor drum from a front side of a mounting space to the inside of the mounting space. The photoreceptor drum is temporarily supported by a repulsive force of a cleaning blade and positioned in a predetermined position in the mounting space. Then, in order to cause a lock member to support a second shaft portion at a second end of the photoreceptor drum, the lock member is retracted against a biasing force of a biasing member. The second shaft portion at the second end of the photoreceptor drum is supported by the lock member by a restoring force of the biasing member, so that the photoreceptor drum is locked in a mounted state.