To provide a workpiece mounting device whereby a material workpiece can be surely clamped by a chuck without applying a large load to a main spindle side, when the material workpiece on a workpiece loading base is received by a spindle side chuck.

A workpiece mounting device 3, provided in a main spindle moving type vertical lathe 1 moving a main spindle base 4 supporting a main spindle 5 having an axial line in a vertical direction able to rotate in at least a main spindle axial line direction and clamping a workpiece able to mount/remove it by a chuck provided in a tip end of the spindle, has a material workpiece mounting part 29 mounting a material workpiece 7 so as to be opposed to the chuck. Here, the material workpiece mounting part 29 has a material workpiece mounting base mounting the material workpiece 7, support part supporting the material workpiece mounting base able to move in the spindle axial line direction also energizing the base to a chuck side of main spindle axial line, and a swivel means 55 pressing a peripheral surface of the material workpiece 7 on the material workpiece loading base by a contact member swiveled to be associated with lowering action of the material workpiece mounting base to position the material workpiece 7 in almost the center position on the material workpiece loading base.