A deceleration device includes a reduction assembly that reduces speed, a motor attached to the reduction assembly, the motor being rotatable about a central axis line, and a heat insulator having a heat conduction coefficient of about 0.04 W/mK or less, the heat insulator being positioned between the motor and the reduction assembly. A surface temperature of an exposed portion of the motor is maintained at a low level, with a simple structure, that prevents burns to human skin in contact with the motor.