PROBLEM TO BE SOLVED: To provide a system for facilitating use of residual hot water by combining a residual-hot-water transfer means and an electric-signal using means, and to provide the electric-signal using means for showing and giving the electric signal on the state of opening/closing of a pop-up plug and for controlling operation of a hot water supplying device, in a bathtub having the pop-up plug with an opening/closing switch. SOLUTION: A residual-hot-water using means 1 is a simple and easy means in which an electromagnetic valve 55 for drain positioned lower than a plug member 28 is closed when the pop-up plug 6 of a residual-hot-water intake port 89 is opened, and the bathtub 70 is communicated with a laundry machine 45 and a toilet 46 to which the residual hot water is transferred. The electric signal from the opening/closing switch 12 of the pop-up plug 6 attached to the residual-hot-water intake port 89 is used for opening and closing the electromagnetic valve 55 for drain. The electric signal also indicates and tells the open/close state of the plug member 28, and is used for controlling in an interlocking manner so that the hot-water supplying device can not be automatically operated in the state that the plug member 28 is opened when the hot-water is poured to fill the bathtub 70. COPYRIGHT: (C)2010,JPO&INPIT