To improve engine performance by conducting suction smoothly, and preventing moisture sucked in an intake air box from being sucked in an internal combustion engine when air is sucked in the internal combustion engine passing through the intake air box and an intake air guiding member.

An intake air guiding member 51 is formed equipped with an intake air passage 58. An intake air box 56 is formed, which is fitted at the upper end of the intake air guiding member 51 to cover an opening at an the upper end of the intake air passage 58. An intake air inlet 72 for communication between its inside and outside is formed at the intake air box 56, and a communication hole 67c is also formed for communicating the inside of the intake air passage 56 with the intake air passage 58 of the intake air guiding member 51. The intake air box 56 is formed so that the internal width dimension W1 in its horizontal direction may be increased at the upper part and decreased at the lower part when viewed from its cross-section. The intake air inlet 72 is formed at the upper part of the intake air box 56, and a drain hole 85 is formed at its lower part.