An automatic tool changer (100A, 100B) comprising: a tool storage magazine (11A, 11B) for storing a tool (TA, TB) to be attached to a main spindle (9A, 9B) of a machine tool; a tool changer magazine (12A, 12B) that is capable of rotating, the tool changer magazine (12A, 12B) being for performing tool replacement with respect to the main spindle (9A, 9B), and with respect to the tool storage magazine (11A, 11B); a cover for partitioning a machining area where the tool changer magazine (12A, 12B) performs tool replacement with respect to the main spindle (9A, 9B), and a non-machining area where the tool changer magazine (12A, 12B) performs tool replacement with respect to the tool storage magazine (11A, 11B); and an opening, provided at the cover, for enabling transfer of a tool (TA, TB) between the tool changer magazine (12A, 12B) and the main spindle (9A, 9B); characterized in that an opening/closing member (17A, 17B) for opening/closing the opening by turning in synchronization with rotation of the tool changer magazine (12A, 12B), wherein the opening/closing member (17A, 17B) includes a plate section (173, 174, 175, 176) for partitioning the machining area and the non-machining area at a position of the opening, the plate section (173, 174, 175, 176) closes the opening by taking a horizontal posture or a vertical posture, in a case where the tool changer magazine (12A, 12B) performs tool replacement with respect to the tool storage magazine (11A, 11B), and the plate section (173, 174, 175, 176) opens the opening by changing a posture from the horizontal posture to an inclined posture or the vertical posture, or from the vertical posture to the inclined posture or the horizontal posture, in a case where the tool changer magazine (12A, 12B) performs tool replacement with respect to the main spindle (9A, 9B). a tool storage magazine (11A, 11B) for storing a tool (TA, TB) to be attached to a main spindle (9A, 9B) of a machine tool; a tool changer magazine (12A, 12B) that is capable of rotating, the tool changer magazine (12A, 12B) being for performing tool replacement with respect to the main spindle (9A, 9B), and with respect to the tool storage magazine (11A, 11B); a cover for partitioning a machining area where the tool changer magazine (12A, 12B) performs tool replacement with respect to the main spindle (9A, 9B), and a non-machining area where the tool changer magazine (12A, 12B) performs tool replacement with respect to the tool storage magazine (11A, 11B); and an opening, provided at the cover, for enabling transfer of a tool (TA, TB) between the tool changer magazine (12A, 12B) and the main spindle (9A, 9B); characterized in that an opening/closing member (17A, 17B) for opening/closing the opening by turning in synchronization with rotation of the tool changer magazine (12A, 12B), wherein the opening/closing member (17A, 17B) includes a plate section (173, 174, 175, 176) for partitioning the machining area and the non-machining area at a position of the opening, the plate section (173, 174, 175, 176) closes the opening by taking a horizontal posture or a vertical posture, in a case where the tool changer magazine (12A, 12B) performs tool replacement with respect to the tool storage magazine (11A, 11B), and the plate section (173, 174, 175, 176) opens the opening by changing a posture from the horizontal posture to an inclined posture or the vertical posture, or from the vertical posture to the inclined posture or the horizontal posture, in a case where the tool changer magazine (12A, 12B) performs tool replacement with respect to the main spindle (9A, 9B).