A ball screw comprising: a threaded shaft having a spiral threaded groove at an outer circumferential surface; a nut having a threaded groove which faces the threaded groove of the threaded shaft at an inner circumferential surface; a plurality of balls rotatably loaded in a spiral ball raceway constituted by both of the threaded grooves ; and a ball circulation groove, which returns and circulates the balls from an end point to a start point of the ball raceway, wherein the nut is formed by concaving a part of the inner circumferential surface of the nut to form the ball circulation groove constituted by a concaved groove, and forming the threaded groove in the inner circumferential surface of the nut to be connected with an end part of the ball circulation groove, and wherein at least a part of an edge portion constituted by both of side surfaces of the ball circulation groove and a surface which continue from each of the side surfaces and which extend in an axial direction is formed to be rounded, characterized in that the ball screw further comprises a lubricant reservoir which can hold a lubricant and the lubricant reservoir is constituted by a dented part formed by denting a part of an inner surface of the concaved groove, wherein the ball circulation groove comprises: both of the end parts connecting with the ball raceway; and an intermediate part between the both of the end parts, and wherein an area of a cross-section of the lubricant reservoir cut along a plane perpendicular to a length direction of the ball circulation groove is larger at a part adjacent to the intermediate part than a part adjacent to the end parts. a threaded shaft having a spiral threaded groove at an outer circumferential surface; a nut having a threaded groove which faces the threaded groove of the threaded shaft at an inner circumferential surface; a plurality of balls rotatably loaded in a spiral ball raceway constituted by both of the threaded grooves ; and a ball circulation groove, which returns and circulates the balls from an end point to a start point of the ball raceway, wherein the nut is formed by concaving a part of the inner circumferential surface of the nut to form the ball circulation groove constituted by a concaved groove, and forming the threaded groove in the inner circumferential surface of the nut to be connected with an end part of the ball circulation groove, and wherein at least a part of an edge portion constituted by both of side surfaces of the ball circulation groove and a surface which continue from each of the side surfaces and which extend in an axial direction is formed to be rounded, characterized in that the ball screw further comprises a lubricant reservoir which can hold a lubricant and the lubricant reservoir is constituted by a dented part formed by denting a part of an inner surface of the concaved groove, wherein the ball circulation groove comprises: both of the end parts connecting with the ball raceway; and an intermediate part between the both of the end parts, and wherein an area of a cross-section of the lubricant reservoir cut along a plane perpendicular to a length direction of the ball circulation groove is larger at a part adjacent to the intermediate part than a part adjacent to the end parts. <EOS>