An exhaust pipe structure has, under a floor of a vehicle, an exhaust passage for exhaust gas discharged from a vehicle-installed engine, and at least one muffler is provided on an intermediate portion of the exhaust passage. The at least one muffler includes a main muffler having the greatest muffler capacity. An external cylinder forming an outer shell portion of the main muffler has a circular sectional shape having a diameter of 100 mm to 150 mm, or a sectional shape that is substantially equal to an area of the circular sectional shape. The external cylinder has a length of 800 mm to 12000 mm. The main muffler is disposed in a tunnel portion that is formed between the front wheels and the rear wheels.