In order to purify NOx in exhaust gas discharged from an engine, a urea water supply system is configured by a urea water injection valve, a urea water tank, urea water supply piping (22) and a urea water supply device (30). Cooling water piping (40) that allows cooling water of the engine to flow through is laid along the urea water supply piping (22), while the cooling water piping (40) is connected to the urea water supply device (30) at a connection portion (34) of an inflow pipe and a connection portion (35) of an outflow pipe, and a part (46) of the inflow pipe is installed to be close to connection portions (31, 32, 33) of the urea water supply piping (22) to the urea water supply device (30) within a predetermined distance range in which heat of the cooling water reaches the connection portions.