Disclosed herein is a hot-water boiler for a hot-water heating mat. The boiler is configured such that a high-temperature space and a low-temperature space, respectively, located on left and right sides of a recess formed in a lower portion of a central region of a hot-water heating tank, a discharge space connecting upper portions of the high-temperature space and the low-temperature space with each other, and a hot-water heating pipe connecting lower portions of the high-temperature space and the low-temperature space with each other are arranged to form a ‘’-shaped circulation path. The hot-water heating pipe is inclined and an electric heater is provided on an outer surface of the hot-water heating pipe.