An image forming apparatus comprising: a drum unit including a photosensitive drum configured to bear a toner image; a developing unit including a developing roller configured to supply toner to the photosensitive drum; an apparatus main body having a mounting space to which the drum unit and the developing unit are independently and detachably attached in an insertion and removal direction; characterized in further comprising: a switching member configured to be movable, with respect to the drum unit and the developing unit, between a first position and a second position, the first position being a position where the switching member is overlapped with the drum unit and is not overlapped with the developing unit when viewed in the insertion and removal direction, the second position being a position where the switching member is overlapped with the developing unit and is not overlapped with the drum unit when viewed in the insertion and removal direction, wherein the switching member includes: a first guide surface configured to guide the developing unit while the developing unit is being mounted to the mounting space in the insertion and removal direction in a case where the switching member is in the first position, the first guide surface being a convex curved surface when viewed in the insertion and removal direction; and a second guide surface configured to guide the drum unit while the drum unit is being mounted to the mounting space in the insertion and removal direction in a case where the switching member is in the second position, the second guide surface being a concave curved surface recessed in a direction in which the first guide surface protrudes when viewed in the insertion and removal direction. a drum unit including a photosensitive drum configured to bear a toner image; a developing unit including a developing roller configured to supply toner to the photosensitive drum; an apparatus main body having a mounting space to which the drum unit and the developing unit are independently and detachably attached in an insertion and removal direction; characterized in further comprising: a switching member configured to be movable, with respect to the drum unit and the developing unit, between a first position and a second position, the first position being a position where the switching member is overlapped with the drum unit and is not overlapped with the developing unit when viewed in the insertion and removal direction, the second position being a position where the switching member is overlapped with the developing unit and is not overlapped with the drum unit when viewed in the insertion and removal direction, wherein the switching member includes: a first guide surface configured to guide the developing unit while the developing unit is being mounted to the mounting space in the insertion and removal direction in a case where the switching member is in the first position, the first guide surface being a convex curved surface when viewed in the insertion and removal direction; and a second guide surface configured to guide the drum unit while the drum unit is being mounted to the mounting space in the insertion and removal direction in a case where the switching member is in the second position, the second guide surface being a concave curved surface recessed in a direction in which the first guide surface protrudes when viewed in the insertion and removal direction. a switching member configured to be movable, with respect to the drum unit and the developing unit, between a first position and a second position, the first position being a position where the switching member is overlapped with the drum unit and is not overlapped with the developing unit when viewed in the insertion and removal direction, the second position being a position where the switching member is overlapped with the developing unit and is not overlapped with the drum unit when viewed in the insertion and removal direction, wherein the switching member includes: a first guide surface configured to guide the developing unit while the developing unit is being mounted to the mounting space in the insertion and removal direction in a case where the switching member is in the first position, the first guide surface being a convex curved surface when viewed in the insertion and removal direction; and a second guide surface configured to guide the drum unit while the drum unit is being mounted to the mounting space in the insertion and removal direction in a case where the switching member is in the second position, the second guide surface being a concave curved surface recessed in a direction in which the first guide surface protrudes when viewed in the insertion and removal direction. a first guide surface configured to guide the developing unit while the developing unit is being mounted to the mounting space in the insertion and removal direction in a case where the switching member is in the first position, the first guide surface being a convex curved surface when viewed in the insertion and removal direction; and a second guide surface configured to guide the drum unit while the drum unit is being mounted to the mounting space in the insertion and removal direction in a case where the switching member is in the second position, the second guide surface being a concave curved surface recessed in a direction in which the first guide surface protrudes when viewed in the insertion and removal direction.