Disclosed is a water supply device installed in a livestock barn for raising livestock such as milk cattle. A water supply unit consisting of an inner case, an outer case and a floating case is installed inward of the topside of a water tank, and a check valve is installed in the floor portion of the floating case, and in the water tank are installed a water supply pipe having a level limit valve, a drainage pipe, an overflow pipe, a heater, a water level sensor and a temperature sensor. By dint of the floating case having a predetermined quantity of water, livestock such as milk cattle can take water by drinking water, and the check valve formed in the floating case plays a role of maintaining the water level in the floating case constant at all times and at the same time it shuts off the inflow of foreign material into the water tank to prevent decomposition of water. Also, the water supply unit on the top of the water tank is open with the least size possible for drinking water, so that the least possible heat loss occurs. Therefore, the wintertime heat-conserving effect is excellent and it is possible to provide water that is clean and of optimum temperature at all times.
