A heating-controller controls an electric power supply from a battery to a urea-water heater based on a voltage of the battery detected by a voltage detector and an engine-state of an internal combustion engine obtained by an engine-state detector. When a voltage of the battery is low or when the internal combustion engine stops, the heating-controller prohibits the electric power supply from the battery to the urea-water heater. A urea-water adding device can avoid an excess decrease of a battery power by intermittently energizing the urea-water heater according to the engine-state of an internal combustion engine and the battery voltage VB.