To effectively reduce the resonance phenomenon of standing wave generated in a cabin by installing a muffler in such a way that an opening part is positioned at the loop part of standing wave in the longitudinal, lateral or vertical direction of a vehicle.

A muffler 1 is installed in such a way that an opening part 1a is positioned on the side far away from a center position of space in a cabin (the rear side of a vehicle) at the time of being installed on a rear board 11. The muffler 1 is therefore installed in such a way that the opening part 1a is positioned at the rear side loop part of standing wave in the longitudinal direction of a vehicle. When a resonance phenomenon caused by engine sound or the like is generated in the cabin with the muffler 1 thus fitted, resonance frequency of the muffler 1 set for the purpose of decrease synchronizes with resonance frequency in the cabin so as to damp the resonance sound. Since the opening part 1a of the muffler 1 is arranged at the rear side loop part of standing wave in the longitudinal direction of the vehicle, resonance sound is effectively damped.