a developing part having a developing process roller to perform a developing process and a position restricting protrusion;

a main body having an installation part at which the developing part is detachably installed and a roller driving part for driving the developing process roller;

a position restricting groove part formed on a predetermined restricting position of the installation part, to engage the position restricting protrusion, so that the developing process roller engages the roller driving part, when the developing part is installed at the installation part;

a first rotation supporting part between the installation part and the developing part, at a first position other than the restricting position, and which supports the developing part to rotate around the first position, when the developing part is rotated along a first circumferential direction of the developing process roller, so that the position restricting protrusion detaches from the position restricting groove part; and

a second rotation supporting part between the installation part and the developing part, at a second position, such that the restricting position is between the first and second positions, the second rotation supporting part supporting the developing part to rotate around the second position, when the developing part is rotated along a second circumferential direction other than the first circumferential direction, so that the position restricting protrusion detaches from the position restricting groove part.