To clean a drain pan by providing a check valve in a drainage pump for draining drain water accumulated in an indoor unit of an air conditioner.

A drainage pump 100 includes a rotary blade 130 inside a pump housing 110, sucks up drain water from a suction port 120, and discharges the drain water from a discharge port 122 to a riser pipe side. A check valve member 200 is freely slidably disposed inside the discharge port 122, a valve is opened by a drain water flow F₁ during an operation, and the valve is closed by being pushed by the drain water returning from the riser pipe during stoppage.