PURPOSE: To prevent the abnormal friction and seizure of a roller or the like by providing a by-pass with one end thereof communicated with a rear muffler and the other end opened into a primary discharge space after by-passing in a front muffler.

CONSTITUTION: Gas discharged onto the rear muffler 10 side is directly discharged into a primary discharge space S1 by a path passing a through hole 11 and a path via a by-pass 12 provided in such a way as to by-pass a front muffler 9. Accordingly, the discharge gas in the rear muffler 10 is partially discharged into the primary discharge space S1 without receiving the influence of gas in the front muffler 9 so as to become small in flow resistance, and also the flow of outflow gas becomes large because of passing both the through- hole 11 and by-pass 12. This results in suppressing pressure rise in the rear muffler 10 and also rise to high temperature.