To maintain surface pressure between an impeller and a thrust surface of a pump chamber proper for a long period of time, to prevent pumping failure and driving loss of the impeller and to reduce manufacturing cost of a displacement type water pump.

This displacement type water pump 17 of an outboard motor is constituted by providing a sliding panel 40 in the same shape as a thrust surface 27 free to move in the thrust direction between the thrust surface 27 on one side of a pump chamber 26 and an impeller 34 and elastically installing a pressuring member 41 to pressurize this sliding panel 40 to the side of the impeller 34 between the thrust surface 27 and the sliding panel 40.