To provide a water supply control device and its system capable of ensuring the consistent water amount and the consistent pressure when operating a plurality of pumps in a changing manner, and safely operating the pumps without any shutoff state.

The water supply control device 13 comprises an adjustment unit 4 for computing the operational amount of a variable speed pump, a number of rotation command unit 5 for computing the control command to be sent to a motor control device 3 of the variable speed motor based on the result of the adjusting unit, and a pump number determination unit 6 for changing the pumps according to the fluctuation of the water supply. The water supply control device controls the number of rotation according to the fluctuation of the water supply while performing the parallel operation of a plurality of pumps and performs the change control of the pumps, and is provided with a speed computation unit 12 which automatically detects the control range of the variable speed pump that the variable speed pump in a variable speed mode and a fixed speed pump in a fixed speed mode are not in a non-water-supply state, and controls the number of rotation when controlling the variable speed pump in a parallel operation.