A turning unit comprising: a turning shaft housed in a housing so that the turning shaft is reciprocable; a belt mechanism having a driving pulley coupled to a motor , a driven pulley disposed on an outer periphery of the turning shaft so that the driven pulley is rotatable, and a belt that is wound around between the driving pulley and the driven pulley ; and a ball screw mechanism that has a ball screw nut coupled to the driven pulley so that the ball screw nut is integrally rotatable, and a plurality of balls disposed in a ball raceway (R1) that has a helical shape and that is provided such that a screw groove provided on the outer periphery of the turning shaft faces a screw groove provided on an inner periphery of the ball screw nut , and that converts a rotation of the ball screw nut into reciprocating movement of the turning shaft , wherein the ball screw nut has a pair of through holes extending through the inner periphery and an outer periphery of the ball screw nut , and a first passage that allows the through holes to communicate with each other and that opens radially outward, the ball screw mechanism has a circulation member having at least one second passage through which the ball can move between the ball raceway (R1) and the first passage in each of the through holes , the ball screw mechanism has a circulation path (R2) for communicating two connection points (P1, P2) set on the ball raceway (R1) by the first passage and the second passage , the driven pulley has, on an inner periphery, a thin portion that defines a radial clearance between the driven pulley and the outer periphery of the ball screw nut , characterized in that the ball screw mechanism has a pressing lid (106; 141) that presses down each of the circulation members from the outer periphery of the ball screw nut and that covers the first passage . a turning shaft housed in a housing so that the turning shaft is reciprocable; a belt mechanism having a driving pulley coupled to a motor , a driven pulley disposed on an outer periphery of the turning shaft so that the driven pulley is rotatable, and a belt that is wound around between the driving pulley and the driven pulley ; and a ball screw mechanism that has a ball screw nut coupled to the driven pulley so that the ball screw nut is integrally rotatable, and a plurality of balls disposed in a ball raceway (R1) that has a helical shape and that is provided such that a screw groove provided on the outer periphery of the turning shaft faces a screw groove provided on an inner periphery of the ball screw nut , and that converts a rotation of the ball screw nut into reciprocating movement of the turning shaft , wherein the ball screw nut has a pair of through holes extending through the inner periphery and an outer periphery of the ball screw nut , and a first passage that allows the through holes to communicate with each other and that opens radially outward, the ball screw mechanism has a circulation member having at least one second passage through which the ball can move between the ball raceway (R1) and the first passage in each of the through holes , the ball screw mechanism has a circulation path (R2) for communicating two connection points (P1, P2) set on the ball raceway (R1) by the first passage and the second passage , the driven pulley has, on an inner periphery, a thin portion that defines a radial clearance between the driven pulley and the outer periphery of the ball screw nut , characterized in that the ball screw mechanism has a pressing lid (106; 141) that presses down each of the circulation members from the outer periphery of the ball screw nut and that covers the first passage . <EOS>