Disclosed herein is a photosensitive body unit having an improved structure capable of disposing a developing unit at a correct position to supply developer to a photosensitive body, and an image forming apparatus including the photosensitive body unit. The image forming apparatus includes a main body, a photosensitive body housing mounted in the main body, a photosensitive body rotatably coupled to the photosensitive body housing, and a developing unit detachably mounted in the main body to supply developer to the photosensitive body. The photosensitive body housing includes a guide portion to determine a mounting position of the developing unit when the developing unit is mounted in the main body.