An image forming apparatus comprising: an image forming unit, including an image bearing member, for rotatably supporting the image bearing member; a developing unit including a first rotatable developer carrying member for developing an electrostatic latent image formed on the image bearing member and a second rotatable developer carrying member for developing the electrostatic latent image formed on the image bearing member; first gap regulating means (14-1-2, 14-1-3), provided at both sides of the first rotatable developer carrying member, for regulating a gap between the image bearing member and the first rotatable developer carrying member; second gap regulating means (14-2-2, 14-2-3), provided at both sides of the second rotatable developer carrying member, for regulating a gap between the image bearing member and the second rotatable developer carrying member; urging means for urging said developing unit toward said image forming unit so that said first gap regulating means (14-1-2, 14-1-3) and said second gap regulating means (14-2-2, 14-2-3) are urged against an abutment portion (11-2-1, 11-2-2, 11-3-1, 11-3-2) provided on said image forming unit; and positioning means (11-2-3, 11-3-3, 14-3-1, 14-3-2) for positioning said developing unit relative to said image forming unit, wherein said positioning means (11-2-3, 11-3-3, 14-3-1, 14-3-2) is provided both on said developing unit and on said image forming unit, characterized in that said positioning means (11-2-3, 11-3-3, 14-3-1, 14-3-2) includes a preventing portion (11-2-3, 11-3-3, 14-3-1, 14-3-2) for preventing, while permitting movement of said developing unit in a circumferential direction of the image bearing member at one longitudinal end of said developing unit, movement of said developing unit in the circumferential direction of the image bearing member at the other longitudinal end of said developing unit. an image forming unit, including an image bearing member, for rotatably supporting the image bearing member; a developing unit including a first rotatable developer carrying member for developing an electrostatic latent image formed on the image bearing member and a second rotatable developer carrying member for developing the electrostatic latent image formed on the image bearing member; first gap regulating means (14-1-2, 14-1-3), provided at both sides of the first rotatable developer carrying member, for regulating a gap between the image bearing member and the first rotatable developer carrying member; second gap regulating means (14-2-2, 14-2-3), provided at both sides of the second rotatable developer carrying member, for regulating a gap between the image bearing member and the second rotatable developer carrying member; urging means for urging said developing unit toward said image forming unit so that said first gap regulating means (14-1-2, 14-1-3) and said second gap regulating means (14-2-2, 14-2-3) are urged against an abutment portion (11-2-1, 11-2-2, 11-3-1, 11-3-2) provided on said image forming unit; and positioning means (11-2-3, 11-3-3, 14-3-1, 14-3-2) for positioning said developing unit relative to said image forming unit, wherein said positioning means (11-2-3, 11-3-3, 14-3-1, 14-3-2) is provided both on said developing unit and on said image forming unit, characterized in that said positioning means (11-2-3, 11-3-3, 14-3-1, 14-3-2) includes a preventing portion (11-2-3, 11-3-3, 14-3-1, 14-3-2) for preventing, while permitting movement of said developing unit in a circumferential direction of the image bearing member at one longitudinal end of said developing unit, movement of said developing unit in the circumferential direction of the image bearing member at the other longitudinal end of said developing unit.