To improve combustion by causing a state of turbulence in a combustion chamber of a cylinder in ignition timing to be suitable.

In an internal combustion engine in which an intake valve provided to an intake port is opened when a piston reciprocating in a cylinder is moved downward and an intake is sucked from the intake port into a combustion chamber of the cylinder, the intake valve is operated so that an opening is expanded again in a period T in which the intake valve is opened to the maximum opening and is then fully closed. In other words, the intake having a speed different from that of an in-cylinder flow generated at an initial stage of an intake stroke is introduced into the cylinder at the latter stage O of the intake stroke so as to collide with the in-cylinder flow. Accordingly, the in-cylinder flow is broken, and is changed into many vortices and turbulences.