PURPOSE: To shorten stand-by time by presetting the residual number of workpieces finished with machining, then comparing the residual number with the present value, and when unmachined material reaches the specified value, moving a transfer means into a carry-out position to store the machined workpieces in a tray.

CONSTITUTION: The numeric value to move a tray T1 upon reaching the specified residual number of workppieces W is preset to the tray T1 positioned on the carry-in side and stored in a workpiece residual quantity set value memory. On the basis of this stored residual quantity, an orthogonal robot 20 is operated, and the residual number and the preset value are compared. When the preset value reaches the set residual number or less and unmachined material M reaches the specified value, the material M is carrired out of the tray T1. On the other hand, workpieces W finished with machining are returned to the tray T1 side for storage from the machine tool side through a temporary placing device 40. Stand-by time can be thereby shortened so as to improve the operating efficiency of the machine tool provided with a tray changer.