A water pump has a pump section having radial impeller blades, a rotational shaft for rotating the radial impeller blades, and a motor section provided in the rear of the pump section and driving the rotational shaft. The motor section has a motor casing, a motor housed in the motor casing and rotating the rotational shaft, bearings provided on both sides of the motor interposed between them on the rotational shaft, interposed between an inner surface of the motor casing and the rotational shaft, and rotatably supporting the rotational shaft. In the motor casing, a vent hole for venting air between a motor space in which the motor is disposed in the motor casing and an outside of the motor casing is formed.