Reduction gear, electric power steering device with reduction gear, and method for manufacturing reduction gear

A reduction gear (22) is provided with a worm shaft (31), first and second ball bearings (33, 34) which rotatably support the worm shaft (31), and a preload application member (35) which applies a preload to the first and second ball bearings (33, 34). The cylindrical section (61) of the preload application member (35) is press-fitted into a first inner peripheral surface (53). The first length (L1) of the first inner peripheral surface (53) is set to be greater than the second length (L2) of the cylindrical section (61). When the preload application member (35) applies a preload to the first and second ball bearings (33, 34), the base end surface (61A) of the cylindrical section (61) is located further toward the front end side than the base end section (53A) of the first inner peripheral surface (53) and at the same time the front end section (61B) of the cylindrical section (61) is located further toward the base end side than the front end section (53B) of the first inner peripheral surface (53).