To provide an internal lubricating ball screw capable of automatically supplying a lubricant stably over a long period of time and realizing good lubrication.

This internal circulation type ball screw is provided with a screw shaft 1 having a ball groove 1b on the outer peripheral surface 1a, a ball nut 2 having a ball screw groove 2b corresponding to the ball screw groove of the screw shaft in the inner peripheral surface 2a, a number of balls 3 rolling along a spiral rolling path between the ball screw groove of the ball nut and the ball screw groove of the screw shaft, and a circulating piece 4 provided on the inner peripheral surface of the ball nut for guiding the ball to go over the thread of the ball screw groove of the screw shaft and circulate to some other ball screw groove. The circulating piece is formed of a sintered body containing lubricating oil.