A water pump includes a mechanical seal provided between a housing and a rotary shaft for preventing cooling water from leaking from a whirl chamber. The housing forms a space into which cooling water leaked from between the mechanical seal and the rotary shaft is flown and a water vent for discharging cooling water flown into the space. The housing is joined with a thermostat cover to form a reservoir communicating with the water vent. The reservoir has a drain for releasing evaporated cooling water to the air and a control wall for preventing cooling water remaining as liquid from flowing out through the drain.