PURPOSE: To accelerate the temperature rise in an exhaust gas purifying device after the starting of an engine in order to activate the device early by a method wherein the activated state of the exhaust gas purifying device is detected and a control valve is kept open until the completely activated state is reached.

CONSTITUTION: When the temperature in the exhaust gas purifying device 31 is higher than that to start activation, a solenoid 14 is turned ON in order to open a solenoid-operated on-off valve 40. When the solenoid-operated on-off valve 40 is open, the chamber 32a of a pneumatic actuator 32 is communicated with a suction air pipe 27 and the chamber 32b is cut-off from a suction air pipe 4 so that a spring 34 pushes a diaphragm 33 in the direction of the chamber 32a in order to block a bypath inlet 24a by means of a control valve 23 and consequently exhaust gas is led through an exhaust gas turbine 22 to the exhaust gas purifying device 31. When the temperature in the exhaust gas purifying device is lower than that to start activation, the solenoid 41 is turned OFF, resulting in causing the exhaust gas discharged from the engine 1 to bypass the exhaust gas turbine 22 so as to flow through the bypath 24 to the exhaust gas purifying device 31.