An exhaust muffler has downstream and upstream chambers which are defined in a casing. Exhaust gas from an internal combustion engine is introduced into an end of a first conduit having a pierced region which is held in fluid communication with the downstream chamber and an end of a second conduit. The other ends of the first and second conduits are open into the upstream chamber. A valve means configured to open/close the first and second conduits is actuated by valve actuating means to switch between a short channel mode discharging the exhaust gas from at least the first conduit through the pierced region, the downstream chamber, and a discharge pipe out of the casing and a long channel mode discharging the exhaust gas from at least the second conduit through the upstream chamber, the pierced region, the downstream chamber, and the discharge pipe out of the casing.