There is provided a rolling element spacer used in a rolling guide device having an endless circulation passage of balls, arranged alternately with many balls in the endless circulation passage, and circulating together with the balls in the endless circulation passage. It has a pair of ball holding seats each of which is formed in a concave spherical face form nearly approximating a spherical face of the ball and sliding-contacts with the ball and, around each ball holding seat, there is formed an annular dropout prevention portion protruding in a ball arranging direction than an edge portion of the ball holding seat and kept in non-contact with respect to the ball seated on the ball holding seat.