To provide a steam pressure drive pump with little loss of steam for driving, a small number of sliding and switching parts and of a long life.

A steam supply pipe 12 is connected to pressure chambers 2, 3 through a three-directional control valve 13. A piston member 4 is arranged in the pressure chambers 2, 3 free to slide, cooling pipes 15, 16 are installed, and they are connected to each other through a cooling liquid supply pipe 20 and a three-directional control valve 19. The piston members moves left and right and performs a pumping function by pressure increase by supply of steam to the insides of the pressure chambers 2, 3 and pressure decrease by condensation by cooling by the cooling pipes 15, 16.