A thin jack includes a body, a piston support post, and a dividing piston. The body has a receiving chamber which opens down, a limiting piston circumferentially disposed on the receiving chamber, and upper and lower passages penetrating the body and being in communication with the receiving chamber. The piston support post is movably received in the receiving chamber and is in fluidtight sliding contact with the limiting piston. The dividing piston is disposed at the piston support post, is in fluidtight sliding contact with the receiving chamber, corresponds in position to the limiting piston to confine the dividing piston to the receiving chamber, and divides the receiving chamber into upper and lower receiving spaces in communication with the upper and lower passages, respectively. Accordingly, elevation of the body is controllable while lifting an object, and users can keep raising the thin jack without moving the thin jack horizontally.