PURPOSE: To obtain a high silencing effect without increasing substantially the passage resistance of the exhaust combustion gas by a method wherein a distance between the side face and the heat receiving face of the resonance type silencer is made smaller only at the vicinity of a small hole than the other part.

CONSTITUTION: The protruding part 2a, formed by protruding a part of an inner tube 2 against the silencer 7, is opposing the small hole 8. The area of a space between the protruding part 2a of the inner tube 2 having the small hole 8 and the silencer 7 is made small, therefore, the silencing effect becomes high while the other part except the protruding part 2a of the inner tube 2 is the same as a conventional type inner tube, therefore, the passage resistance of the exhaust gas of combustion is not increased substantially and it is not necessary substantially to make the size of a fan 6 bigger. When the diameter of the small hole 8 is shown by a notation (d) and the length of said protruding part is shown by L, it is sufficient if L is larger than 2d.