To provide an inexpensive and compact support structure of a ball screw shaft without impairing reliability and durability.

In this support structure of the ball screw shaft, a ball screw shaft 1 is rotatably supported on a housing through a rolling bearing 3. A fitting part 3 is formed on the end part of the ball screw shaft 1 to have a smaller diameter than the core diameter of a thread groove of the ball screw shaft 1, and the rolling bearing 3 is pressed in the fitting part 2 with a predetermined interference so that the maximum hoop stress of an inner ring 7 is 180 N/mm2 or less. The support structure includes the rolling bearing 3 and a fixed ring 5 mounted on the fitting part 2, wherein an annular groove 4 having a predetermined angle α of inclination so that the side wall 4a of the end face side is flared toward the outside diameter side is formed on the fitting part 2, and the fixed ring 5 formed of a rod material having a circular section, whose line diameter is a little larger than an annular clearance between the side wall 4a and the inside diameter chamfer 7a of the inner ring 7 is mounted in the annular groove 4.