A temporary tightening tool for a fastening member, which is used for temporarily tightening a fastening member that is a bolt or nut for fastening a tire wheel to a hub, comprising: a grip part (10, 100, 300) for an operator to input torque, and a socket part (20, 200, 400) formed at a tip of said grip part, into which a hexagonal columnar part formed at said fastening member can be inserted, wherein said socket part comprises: a leaf spring part (42, 52, 405) which is pressed by a part of six side surfaces of said hexagonal columnar part to be elastically deformed outward in a radial direction of said hexagonal columnar part when said hexagonal columnar part is inserted in said socket part, and presses said part of said six side surfaces inward in the radial direction with restoring force to hold said hexagonal columnar part such that said hexagonal columnar part is clamped, characterised in that said socket also comprises a torque transmission part (30,404) which transmits torque to a side surface that is not pressed by said leaf spring part among said six side surfaces of said hexagonal columnar part when said torque is input to said grip part in a state where said hexagonal columnar part has been inserted in said socket part, and in that said socket part (20, 200) is formed in a shape of a hexagonal tube surrounded by six side walls, two slits (41, 51) are formed a predetermined dimension away from each other in a width direction to reach a tip of said side wall along an axis direction in each of alternate three side walls among said six side walls, said leaf spring part (42, 52) is a plate body formed between said two slits, and said torque transmission part is prepared in each of three remaining side walls without said slits among said six side walls, in which a thick part with plate thickness thicker than said leaf spring part is formed. a grip part (10, 100, 300) for an operator to input torque, and a socket part (20, 200, 400) formed at a tip of said grip part, into which a hexagonal columnar part formed at said fastening member can be inserted, wherein said socket part comprises: a leaf spring part (42, 52, 405) which is pressed by a part of six side surfaces of said hexagonal columnar part to be elastically deformed outward in a radial direction of said hexagonal columnar part when said hexagonal columnar part is inserted in said socket part, and presses said part of said six side surfaces inward in the radial direction with restoring force to hold said hexagonal columnar part such that said hexagonal columnar part is clamped, characterised in that said socket also comprises a torque transmission part (30,404) which transmits torque to a side surface that is not pressed by said leaf spring part among said six side surfaces of said hexagonal columnar part when said torque is input to said grip part in a state where said hexagonal columnar part has been inserted in said socket part, and in that said socket part (20, 200) is formed in a shape of a hexagonal tube surrounded by six side walls, two slits (41, 51) are formed a predetermined dimension away from each other in a width direction to reach a tip of said side wall along an axis direction in each of alternate three side walls among said six side walls, said leaf spring part (42, 52) is a plate body formed between said two slits, and said torque transmission part is prepared in each of three remaining side walls without said slits among said six side walls, in which a thick part with plate thickness thicker than said leaf spring part is formed. a leaf spring part (42, 52, 405) which is pressed by a part of six side surfaces of said hexagonal columnar part to be elastically deformed outward in a radial direction of said hexagonal columnar part when said hexagonal columnar part is inserted in said socket part, and presses said part of said six side surfaces inward in the radial direction with restoring force to hold said hexagonal columnar part such that said hexagonal columnar part is clamped, characterised in that said socket also comprises a torque transmission part (30,404) which transmits torque to a side surface that is not pressed by said leaf spring part among said six side surfaces of said hexagonal columnar part when said torque is input to said grip part in a state where said hexagonal columnar part has been inserted in said socket part, and in that said socket part (20, 200) is formed in a shape of a hexagonal tube surrounded by six side walls, two slits (41, 51) are formed a predetermined dimension away from each other in a width direction to reach a tip of said side wall along an axis direction in each of alternate three side walls among said six side walls, said leaf spring part (42, 52) is a plate body formed between said two slits, and said torque transmission part is prepared in each of three remaining side walls without said slits among said six side walls, in which a thick part with plate thickness thicker than said leaf spring part is formed.