Image forming apparatus including detachable drum unit

An image forming apparatus comprising:

• a developing device including a developing roller;
• a drum unit which includes a photosensitive drum having an outer circumferential surface facing an outer circumferential surface of the developing roller each other and has a protrusion protruding in an axial direction of the photosensitive drum;
• a side plate facing an end surface of the drum unit in the axial direction;
• an upwardly open notch having a pair of side portions facing each other, and provided in the side plate at an attachment position of the drum unit;
• a turning member having a curved portion protruded in an arc shape toward the notch and a planar inclined portion connected to a lower end portion of the curved portion, and capable of turning in a direction protruding into the notch and in a direction separating from the notch along a direction crossing the axial direction;
• and
• a biasing member which biases the turning member in a direction close to the notch, wherein
• the turning member is pushed back when the protrusion inserted into the notch from above comes into contact with the curved portion, and when the protrusion reaches a bottom portion of the notch, the inclined portion of the turning member presses the protrusion against one of the side portions and the bottom portion of the notch by a load containing a downward component, and
• the developing device is biased in a direction to bring the developing roller closer to the photosensitive drum, and when the protrusion reaches the bottom portion of the notch, the inclined portion applies a load in a direction crossing a direction in which the developing device is biased, to the protrusion.