To provide a water supply device for a flush toilet that is producible at low cost and capable of saving water by using remaining hot water in a bathtub as much extent as possible.

A water supply device 1 is constructed of a low tank 3 for supplying running water through a valve 31, a flush toilet bowl 2 supplied with a fixed amount of water from the low tank 3, and a water tank 4 for pumping up the remaining hot water from a bathtub 7 and storing the pumped-up hot water. As the water tank 4 is made larger so that supplying can be made from the water tank 4 to the low tank 3 with a pump 5, the remaining hot water of the bathtub 7 cab be stored in as much amount as possible, and furthermore the pump 5 can be used for storing rainwater in a rainwater tank 6 and therefore equipment can be installed at low cost while discharge to a sewer can be restrained.