In an outer ring, a first ball filling slot which extends from a first raceway groove to a first end face of one side of an axial direction is formed, and in an inner ring, a second raceway groove and a second ball filling slot which extends from the second raceway groove to a second end face of one side of the axial direction are formed. Circumferential extension grooves which extend from one of the first ball filling slot and the second ball filling slot by a predetermined length in a circumferential direction at the opposite sides of the circumferential direction of the ball filling slot and extend from the first raceway groove to the first end face in the axial direction are formed.