A muffler (1) having a flat cross-sectional shape which causes the width (W) of the muffler (1) to be greater than the height (H) thereof is provided with: expansion chambers (4-6) therein; and an inlet pipe (7), an outlet pipe (8), and first to third communication pipes (9-11), the pipes (7, 8, 9-11) opening the expansion chambers (4-6) to the outside. An opening of the inlet pipe (7), the opening being formed within the muffler (1), an opening of the outlet pipe (8), the opening being formed within the muffler (1), and the openings of each of the first to third communication pipes (9-11), the openings being formed within the muffler (1), are all arranged so as to be located between a position which is separated from one end of the muffler (1) in the width direction thereof by a distance of one fourth of the width (W) of the muffler (1) and a position which is separated from the one end by half the width (W).