PURPOSE: To reduce a burden to an engine by reducing a frequency component of exhaust noise in a wide range, and also preventing a rise in back pressure at high rotation time of the engine.

CONSTITUTION: In a silencer 69 in which a resonance chamber is arranged by partitioning the inside of an outer cylinder 45, whose both end opening parts are blocked up by end plates 47 and 49, by partition plate 53 in which a resonance tube 67 is inserted and installed, a bypass pipe line 71 opening outside of the outer cylinder 45 is arranged in the resonance tube 67 at a desired distance from a resonance chamber side end part 67a of the resonance tube 67, and a control valve 73 to open and close the bypass pipe line 71 according to rotating speed of an engine is arranged in the bypass pipe line 71.