An image forming apparatus comprising: a main body; a photosensitive unit comprising a photoreceptor; a developing unit including a developing roller which is in contact with the photoreceptor, on which an electrostatic latent image is formed, to supply a developer and is movable away from the photoreceptor; at least one pressing unit configured to bring the developing roller and the photoreceptor in contact; an operating unit which is movable between an operating position at which a pressing force of the pressing unit is generated, and a standby position at which the pressing force is released; and a cover unit provided on one side of the developing unit and the photosensitive unit and configured to guide movement of the operating unit to move the operating unit to the operating position; characterized in that the operating position comprises a fixed operating position (O1a) in which the pressing unit is continuously positioned at a pressing position and a moveable operating position (O1b) in which the pressing unit is moveably positioned at said pressing position, where when the pressing unit is at the pressing position the developing roller and the photoreceptor are adhered to each other; and in that the cover unit is detachably provided and comprises a guide rib having a first guide rib and a second guide rib whereby when the operating unit is positioned at the fixed operating position (O1a), the operating unit is moved to the movable operating position (O1b) under the guidance of the first guide rib when the cover unit is mounted on said one side and when the operating unit is positioned at the standby position, the operating unit is moved to the movable operating position (O1b) under the guidance of the second guide rib when the cover unit is mounted on said one side. a main body; a photosensitive unit comprising a photoreceptor; a developing unit including a developing roller which is in contact with the photoreceptor, on which an electrostatic latent image is formed, to supply a developer and is movable away from the photoreceptor; at least one pressing unit configured to bring the developing roller and the photoreceptor in contact; an operating unit which is movable between an operating position at which a pressing force of the pressing unit is generated, and a standby position at which the pressing force is released; and a cover unit provided on one side of the developing unit and the photosensitive unit and configured to guide movement of the operating unit to move the operating unit to the operating position; characterized in that the operating position comprises a fixed operating position (O1a) in which the pressing unit is continuously positioned at a pressing position and a moveable operating position (O1b) in which the pressing unit is moveably positioned at said pressing position, where when the pressing unit is at the pressing position the developing roller and the photoreceptor are adhered to each other; and in that the cover unit is detachably provided and comprises a guide rib having a first guide rib and a second guide rib whereby when the operating unit is positioned at the fixed operating position (O1a), the operating unit is moved to the movable operating position (O1b) under the guidance of the first guide rib when the cover unit is mounted on said one side and when the operating unit is positioned at the standby position, the operating unit is moved to the movable operating position (O1b) under the guidance of the second guide rib when the cover unit is mounted on said one side.