A DPF and a SCR catalyst are provided in an exhaust pipe, and a urea solution addition valve is provided between the DPF and the SCR catalyst for performing addition supply of urea solution into the exhaust pipe. In a urea solution supply system, the urea solution of prescribed concentration is stored in a urea solution tank, and a urea solution pump is provided in the tank. A urea solution supply pipe is connected to the urea solution pump. A part of the urea solution supply pipe is structured as a compound pipe structure defining two passages. One of the two passages serves as a urea solution passage, and the other one serves as an engine coolant passage.