The present invention discloses a structure of supporting an image bearing member in which the tip portions of support pins inwardly projecting from side plates of a housing, come in contact with the bottoms of support-pin introducing holes in the image bearing member, so that tension applied to a grid electrode disposed opposite to the image bearing member is received by rigidity obtained by the image bearing member and the support pins. Predetermined gaps are formed between the side plates and the flange portions of the image bearing member.