To surely purify the exhaust gas during the work while a working vehicle is stopped and prevent air pollution by branching the discharge pipe of the exhaust gas into an exhaust gas purifying pipe arranged with a gas purifying catalyst unit and an exhaust pipe via a selector valve, and switching the selector valve in response to the actions of a parking brake and a power extracting device.

When a work automobile is stopped and a parking brake lever 31 is operated to start work, a relay 32A is turned on. When a switch 8 is turned on to operate a power extracting device 9, a solenoid valve 6B is opened to start the operation of the power extracting device 9, and a relay 32C is tuned on. When these relays 32A, 32C are concurrently turned on, a solenoid valve selector valve 6A is opened, compressed air is fed to a cylinder device 5, and a piston 23 is extended. A selector valve 2 is moved, the discharge pipe of the exhaust gas is switched to an exhaust gas purifying pipe side, the exhaust gas of an engine 1 is purified by a gas purifying unit 3, then it is exhausted via a muffler 4.