To provide an underground based pump unit to be set on the way of a conduit for rainwater, etc., for discharging water from an outflow pipe on the discharge side to a river, etc.

A concrete tank 3 to which a short pipe 4 on the inflow side and a discharge pipe 5 are connected is manufactured, and the concrete tank 3 is buried underground. The short pipe 4 on the inflow side and the discharge pipe 5 of the concrete tank 3 are jointed with an inflow pipe 6 communicating with a conduit 1 such as a storm sewer and an outflow pipe 7 communicating with a river A, etc. Blocks 8a are stacked on top of the concrete tank 3 to make a manhole 8, and a detachable horizontal shaft submerged pump 9 is disposed in the concrete tank 3. The pump unit can be set up only in a period of time for the assembly on the spot, so that the construction period can be reduced. In addition, as it is not necessary to obtain a wide site for the pump unit, the pump unit is applicable to a pump facility which needs the pump urgently. Further, the horizontal shaft submerged pump can be lifted up on the ground and installed on a detachable device, so that the inspection and repair on the ground are possible.