PURPOSE: To facilitate reduction of the temperature of intake air to a proper value and to increase an amount of intake air flowing in a cylinder by a method wherein an intake air pipe formed by a material having low thermal transmissivity is used or the outside of the intake air pipe is covered with a heat insulating material or an intake air pipe having double structure is used.

CONSTITUTION: A plurality of intake air pipes 1 to a part of each of which a throttle valve 8 is mounted are fastened against respective cylinder heads 3, each having an intake air hole, by means of respective fastening tools 4 to form an intake air passage 9 on the whole. A cylinder head 3 is mounted to a cylinder block 6 to form a combustion chamber 10 in association with a piston 7. The intake air passage 9 is connected to the combustion chamber 10 through an intake air valve 5. In this case, a resin material having low thermal transmissivity is used for the one intake air pipe 2 mechanical strength of which may be low. This constitution prevents the increase of the temperature of the intake air pipe 2 due to a high temperature heat flow from the cylinder head 3 and high temperature air around an engine, and facilitates reduction of the temperature of intake air in the intake air passage 9 to a proper value.