Clogging of an injection nozzle which supplies a reducing agent to the exhaust gas on an upstream side of a reduction catalyst in the exhaust gas passage is prevented, and the efficiency of NOx purification processing is improved. A reducing agent supply unit uses a detection signal of the exhaust gas temperature from a temperature detection device to set a supply quantity at or above a lower limit for cooling the interior of an injection nozzle to below the temperature at which urea water crystallizes, for the detected exhaust gas temperature, and supplies urea water to the injection nozzle at the set supply quantity. By such supply of urea water, the interior of the injection nozzle is cooled to below the temperature at which the urea water crystallizes. As a result, the urea water does not crystallize inside the injection nozzle, and clogging of the injection nozzle can be prevented.