To provide a reduction gear capable of easily controlling pre-load amount of a rolling bearing and an electric power steering device equipped with the same, and to provide a method of manufacturing the reduction gear.

The reduction gear 22 includes: a worm shaft 31 accommodated in a housing 23; a first ball bearing 33 and a second ball bearing 34 mounted on this worm shaft 31; and a pre-loading member 35, which gives the pre-load to each of the ball bearings 33, 34. When a tip end surface 62A of a contact portion 62 of the pre-loading member 35 contacts a first contact surface 53 of the housing 23 and a base end surface 33D of the first ball bearing 33 and when a tip end surface 34D of the second ball bearing 34 contacts a second contact surface 54 of the housing 23, the adjustment of the pre-load of each of the ball bearings 33, 34 are finished.