An image forming apparatus includes a sheet feeding device (1). The sheet feeding device (1) includes a sheet feeding cassette (2), a push-up member (4), a drive shaft (5), and a rotation detecting device (6). The rotation detecting device (6) includes an output gear connected to the drive shaft (5). The output gear includes a gear main body provided with a gear tooth on its outer edge; and an electrode holding member that is attached to the gear main body. The electrode holding member is rotatably provided in the case (9) and a rotating electrode is attached to the electrode holding member. The electrode holding member is formed of at least one thermoplastic resin selected from the group consisting of POM, PA, PBT, PP, PE, ABS resin, PS, PPE, PC, and PMMA. The gear main body is formed of a material whose strength is higher than thermoplastic resin of which the electrode holding member is formed.