A ball screw, comprising: a nut having a spiral groove formed on an inner peripheral surface; a screw shaft having a spiral groove formed on an outer peripheral surface; balls arranged between a raceway formed by the spiral groove of the nut and the spiral groove of the screw shaft ; and a ball return passage configured to return the balls from an end point of the raceway to a start point of the raceway, wherein the balls roll within the raceway and the screw shaft moves in an axial direction when the nut rotates, wherein the nut includes a cylindrical portion , a flange portion formed at a center of the cylindrical portion and a protruding portion configured to protrude from an axial end surface, the protruding portion being arranged at a part of the nut in a circumferential direction, wherein the ball return passage is a recessed portion formed on the inner peripheral surface of the nut , and wherein a center of gravity of the nut deviates from a rotational center line due to the protruding portion , and deviation is made smaller by the recessed portion; characterized in that the nut includes two recessed portions, and wherein the two recessed portions are arranged at the positions where central angles with the protruding portion being as a reference become 90 degrees or less in the circumferential direction of the nut ; and in that the nut and the protruding portion are made of metal, wherein each recessed portion is formed by a piece being fitted into a through-hole passing through the nut in a radial direction, wherein the piece is made of synthetic resin; wherein the through-hole is formed at a position including the flange portion in an axial direction. a nut having a spiral groove formed on an inner peripheral surface; a screw shaft having a spiral groove formed on an outer peripheral surface; balls arranged between a raceway formed by the spiral groove of the nut and the spiral groove of the screw shaft ; and a ball return passage configured to return the balls from an end point of the raceway to a start point of the raceway, wherein the balls roll within the raceway and the screw shaft moves in an axial direction when the nut rotates, wherein the nut includes a cylindrical portion , a flange portion formed at a center of the cylindrical portion and a protruding portion configured to protrude from an axial end surface, the protruding portion being arranged at a part of the nut in a circumferential direction, wherein the ball return passage is a recessed portion formed on the inner peripheral surface of the nut , and wherein a center of gravity of the nut deviates from a rotational center line due to the protruding portion , and deviation is made smaller by the recessed portion; characterized in that the nut includes two recessed portions, and wherein the two recessed portions are arranged at the positions where central angles with the protruding portion being as a reference become 90 degrees or less in the circumferential direction of the nut ; and in that the nut and the protruding portion are made of metal, wherein each recessed portion is formed by a piece being fitted into a through-hole passing through the nut in a radial direction, wherein the piece is made of synthetic resin; wherein the through-hole is formed at a position including the flange portion in an axial direction. <EOS>