This workpiece support device for a machine tool is provided with: a rotary shaft that supports a workpiece and that can rotate about a first axis; a first support device that has a through-hole portion capable of supporting a guide bush adapter and that can receive the lead end portion of the rotary shaft in the through-hole portion; a second support device that supports the rotary shaft and moves the rotary shaft relatively to the first support device in a direction parallel to the first axis; and an attachment that is disposed to be brought into contact with the external surface of the lead end portion of the rotary shaft or the internal surface of the through-hole portion. At least a portion of the attachment disposed on the rotary shaft can be displaced toward a direction away from the first axis, or at least a portion of the attachment disposed on the through-hole portion of the first support device can be displaced toward a direction approaching the first axis.