A ball screw device 10 capable of increasing a load capacity by increasing the size of ball diameter and the number of threads by preventing the cutout of a nut from being interfered with a thread groove adjacent to the cutout even in the thread groove of a small lead screw and a multiple thread screw, wherein a circulating block 18 forming, in association with the cutout 17, a ball 15 circulation passage 21 for communicating an area between both thread grooves 11 and 13 with a ball return passage 16 is fitted to the cutout 17 formed at the end of the nut 14 so that the large number of balls 15 can be endlessly circulated between both thread grooves 11 and 13, ball circulation passage 21, and ball return passage 16. The face of the cutout 17 facing the axial direction thereof is used as a ball running surface 23, a ball circulating groove 20 is formed at the portion of the circulation block 18 opposed to the ball running surface 23, and the ball circulation passage 21 is formed of the ball circulating groove 20 and the ball running surface 23.