To provide a deflector type ball screw capable of improving fixing strength of a deflector member to a deflector window by minimizing clearance determined by a dimensional difference between the deflector member and the deflector window without impairing insertability of the deflector member, and regulating movement of the deflector member in the deflector window to a minimum.

In the deflector type ball screw equipped with the deflector member 5 made of metal fit in the deflector window 6 bored in a body part of a nut 3, a portion from a nut inner diameter side end of the deflector window 6 to an axial outer diameter middle position of the deflector window 6 is an enlarged hole part 61 with a larger clearance (a gap) C with the deflector member 6 than a portion of a nut outer diameter side of the deflector window 6, the portion of the nut outer diameter side of the deflector window 6 is a reduced hole part 62 reducing the clearance (the gap) C with the deflector member 6 than the enlarged hole part 61, and a boundary part between the enlarged hole part 61 and the reduced hole part 62 is a taper part 63 gradually becoming smaller from the enlarged hole part 61 toward the reduced hole part 62.