A spur gear deceleration mechanism unit (30) of a wrist structure unit is adapted to be driven in a first deceleration ratio structure or a second deceleration ratio structure. The first deceleration ratio structure uses at least a first spur gear (31) fitted onto an output shaft of a wrist inner frame driving motor (21) and a second spur gear (32) rotatably supported by a first auxiliary shaft (41). The second deceleration ratio structure uses at least the first spur gear, a first additional spur gear (51) rotatably supported by an additional auxiliary shaft (50), and a second additional spur gear (52) rotatably supported integrally with the first additional spur gear by the additional auxiliary shaft.