To provide a booster and a press forming machine with it, which is able to put forth larger working force while ensuring comparatively large stroke.

A first cylinder 21 and a second cylinder 22, whose cross section is larger than the first one, are connectedly formed into one body, being aligned in the same axis. A first piston 23 and a second piston 24 are inserted and slid in the first cylinder and the second cylinder respectively. A projection 25, shorter in the axial direction than the stroke of the first piston, is mounted on the second piston. The projection is inserted in the first cylinder tightly but slidingly. The move of the first piston puts forth larger working force than that of the first piston to the second piston through the medium of liquid in the first cylinder and the second cylinder.