PURPOSE: To obtain a smaller and simpler underwater pump which has a driving unit capable of taking any installation place and can take a high lift head by a method wherein the driving unit of the under water pump body is operated in connection to a pipe etc. filled with pressure transmitting fluid.

CONSTITUTION: The pump body 1 is composed of cylinders 11, 12 arranged in series, pistons 13, 14 reciprocating in each cylinder 11, 12 and a connecting rod 15 connected to the pistons. Both chambers pinching the piston 14 in one cylinder 12 are connected respectively to each driving cylinder 21, 21 of the driving unit 2 through pipes 18, 18 filled with pressure transmitting fluid and a pair of water supply valves 16, 16, 17, 17 are provided on the other cylinder 11. In this way, since in the pump body 1 placed in the water the driving unit 2 is connected by the pipes 18, 18 etc. filled with pressure transmitting liquid and the pump body 1 is operated by the pressure transmitting fluid, the driving unit 2 can take any installment position and a high lift head. As a result, the unit can be made smaller and simpler.