A projection amount of rod can be adjusted, and workpiece with various shapes or easily deformable workpiece can be stably supported. When an opening/closing valve is switched to an opening position and a working medium flows and is injected from one medium chamber to another medium chamber, a support rod moves upward until an upper face of a first piston abuts on a cover member. Next, when the workpiece is placed on the support rod, the support rod is pushed down. When the workpiece moves downward to a predetermined height, the working medium is prevented from flowing between one medium chamber and another medium chamber if the opening/closing valve is switched to the closing position. Therefore, movement of the support rod is stopped. Since the working medium is sealed in the workpiece support device, it does not leak to the outside.