PURPOSE: To improve the output under a specific operating area of engine by arranging a pressure wave reflector for squeezing half of the intake air at a open end section of an air chamber disposed in the upstream of an intake tube and producing positive and negative pressure waves at said open end section so as to relieve the pressure pulsation.

CONSTITUTION: A reflector 44 having such area as to squeeze half of an intake tube 30 is provided at an open end for an enlarging chamber or an air chamber 32 of an air cleaner disposed in the upstream of the intake tube 30. A pulsating intake air produced in the intake tube 30 reflects at an open end 34 of the air chamber of the intake tube 30 and a negative pressure wave at the open end returns as a positive pressure wave with an atmospheric air flowing therein. Here a negative pressure wave colliding against the reflector 44 returns as it is to an intake port 26 side. Said positive and negative pressure waves are cancelled thereby high output can be obtained without influence onto the fuel to be sucked through a fuel nozzle of a carburetor by the pressure wave.