To provide a relay pump unit to be set on the way of an underground conduit for rainwater, etc., for changing the direction of flow to another conduit with more capacity according to the water level of the conduit.

An inflow pipe 5 branched from a conduit A for rainwater, etc., and an outflow pipe 6 for discharging water to another conduit B1, etc., are connected to a cylindrical concrete tank 2, and a pump gate 8 supporting a horizontal shaft submerged pump 9 is disposed in the concrete tank 2. A rotation drive unit 20 is provided with the horizontal shaft submerged pump 9. Communication of a suction opening 9a and a discharge opening 9b of the horizontal shaft submerged pump 9 with the inflow pipe 5 and the outflow pipe 6 is freely switched. The inflow or outflow from the plural conduits A and B1 is controlled by rotating the horizontal shaft submerged pump 9 according to the water level of the conduits A and B1 into which rainwater, etc., flows, and discharged rainwater, etc., can be made to flow in reverse into the conduit with more capacity.