An improvement to a ball screw device comprising a nut member and a screw shaft screwed to each other via many endlessly-circulating balls, wherein balls are arranged rollably on a band-shaped ball alignment member at a specified interval and in rows and circulate through an endless circulating passage together with the ball alignment member when the nut member and the screw shaft rotate relative with each other. Guide pieces extend outwardly from the both sides of the ball alignment member and guide grooves are formed along the entire circumference of the endless circulating passage for receiving the guide pieces, therefore, the ball alignment member can circulate through the endless circulating passage without wobbling while guided by the guide grooves.