PURPOSE: To simplify a structure for increasing an effective sectional area when a fluid passage is opened, and reduce energy loss.

CONSTITUTION: An intake valve 13 has a first member 14, a second member 15 and a coil spring 17. The first member 14 is periodically pressed down by means of a cam shaft 25. The second member 15 is connected to the first member 14 in a relatively movable manner, in contact with a valve seat 12 for closing an intake port 9, or separated from the seat 12 for opening the intake port 9. A coil spring 17 is arranged between the members 14 and 15, and energizes he second member 15 so as to approach the first member 14. The spring 17 regulates relative movement of the second member 15 when the first member 14 is driven with a specified speed or lower by means of the cam shaft 25. When it is driven with a speed exceeding the specified value, the second member 15 is allowed to relatively move in the direction of separating from the first member 14 by inertia force and increases an effective sectional area of the intake port 9.