To ensure higher quietness while avoiding increase of size of a compact reduction gear in a drive unit having the reduction gear used in an actuator of an industrial robot.

This planetary gear reduction gear is provided with a sun gear 14 connected with an input shaft 3 integrally, a plurality of planetary gears 15 revolving while rotating around the sun gear 14, a ring gear 16 meshing with the planetary gears 15 and fixed to a housing 2, and an output shaft 5 rotating centered on a rotary shaft of the sun gear 14 by the revolution of the planetary gears 15. Each gear 14 to 16 is formed into helical gear. The total meshing rate in meshing of the sun gear 14 and the planetary gears 15 is set in a range of 2.0 to 3.5, and crowning is applied to at least the planetary gears 15.