An object is to make it possible to thaw a urea aqueous solution, which is frozen in a reducing agent tank 230 of an exhaust purification device, more rapidly. When a urea aqueous solution in a reducing agent tank 230 is frozen at a time of starting an engine, at least a part of the urea aqueous solution to be supplied to an injection nozzle 200 is flown into a reducing agent circulation flow path 256, a part of which is adjacent to a heat source, and the urea aqueous solution thus heated is returned to the reducing agent tank 230.