A torsion spring has a coil portion and a leg portion extending from the coil portion. A pin is inserted into the coil portion of the torsion spring in a first position so that the torsion spring is supported by the pin for rotation about the pin. The pin is moved to bring the torsion spring to a second position. The leg portion of the torsion spring is caused to abut against a guide member so that the torsion spring is rotated by a predetermined angle about the pin during the movement from the first position to the second position.