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 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 (101, 102) extending through the inner periphery and an outer periphery of the ball screw nut, and a first passage that allows the through holes (101, 102) 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 and the first passage in each of the through holes (101, 102), the ball screw mechanism has a circulation path for communicating two connection points (P1, P2) set on the ball raceway 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 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 (101, 102) extending through the inner periphery and an outer periphery of the ball screw nut, and a first passage that allows the through holes (101, 102) 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 and the first passage in each of the through holes (101, 102), the ball screw mechanism has a circulation path for communicating two connection points (P1, P2) set on the ball raceway 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.