To control the operation of a water supply pump so as to switch a water for mixing a concrete from a river water to a treated water automatically before the river water gets muddy after perceiving a rainfall.

This device has a first pump 3G for sending river water to a water supply tank 6 in which water for using as water for mixing a concrete is stored and a second pump 5D for sending a treated water. In this case, when a precipitation detector 11 detects a precipitation of prescribed level or more, the timer on the control unit 9 is operated and a river water supply operation by the first pump 3G is stopped and only the treated water supply operation by the second pump 5D is carried out. When the precipitation is at a prescribed level or less, the river water supply operation is started by the first pump 3G after delaying by the set time of a timer.