PURPOSE: To enhance the filling efficiency in an engine by enhancing the flow coefficient through prevention of swirls generated in an intake port, and maintaining high the flow speed and amount of intake gas.

CONSTITUTION: In a straight intake passage 3 there is provided such a constitution that a guide 11 protruding along the direction of intake flow is arranged downstream of a stem guide 8 for guiding the stem 7 of an intake valve, an end face of the upstream part of the guide 11 is brought into close contact with the stem guide 8, and a downstream part of the guide 11 extends downstream while presenting an arcute ridge line, and intake flow is turned to move toward the opening of a combustion chamber 4 from the downstream range of the stem guide 8.