To provide a ball screw improved in service life, operating accuracy, and silentness and a motor-driven cylinder having the ball screw.

This ball screw 1 comprises steel balls each having a surface coated with a DLC film having excellent hardness, slidability, rollability, and wear resistance, a screw shaft 11, and a nut 12. The steel balls roll between a screw groove 111 formed in the outer peripheral surface of the screw shaft 11 and the inner peripheral surface of the nut 12. Therefore, the steel balls are prevented from being deformed, damaged, or worn due to the contact with the screw shaft 11, the nut 12, or the adjacent steel ball. The operating efficiency of the steel balls is maintained since the deterioration is suppressed by the coating of the DLC film thereon. Consequently, since the performance of the ball screw 1 as well as the performance of the motor-driven cylinder 2 with the ball screw 1 are prevented from being lowered due to the deterioration of the steel balls, the service life, the operating accuracy, and the silentness of the ball screw 1 and the motor-driven cylinder are enhanced.