A speed reduction mechanism (30) includes a sun gear (31) that consists of a spur gear； three planetary gears (32)； a fixed gear (33)； and a movable gear (34). The movable gear (34) has internal teeth (34a) on an inner circumferential face side and has external teeth (34b) on an outer circumferential face side. The number of internal teeth (33a) of the fixed gear (33) differs from the number of internal teeth (34a) of the movable gear (34) by a predetermined number. The fixed gear (33) functions as the fixed gear (33) when an engaging portion (51a) is engaged with an engaging groove (33b). External teeth (32a) of the planetary gear (32) engages with external teeth (31a) of the sun gear (31) and the internal gears (33a, 34a) of the fixed gear (33) and the movable gear (34).
