To provide a reduction gear capable of shortening a period of time required to adjust pre-load 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 includes ball bearings 33, 34 which support a worm shaft 31 rotatably, and a pre-loading member 35 which gives the pre-load to each of the ball bearings 33, 34. The pre-loading member 35 has a body 61 mounted on a housing 23 and claws 62 connected to the body 61 in the radial direction. Connection portions 65 of the body 61 with the claws 62 and the body 61 are provided outside from the first ball bearing 33 in the radial direction. The claws 62 are formed so as to be deformable elastically using the connection portions 65 as fulcrums and held in an elastically deformed state by the contact with the first ball bearing 33. Then, a force that the claws 62 tend to restore the state before elastic deformation gives the pre-load to each of the ball bearings 33, 34.