To solve the problem wherein the effect of non-throttle operation on improved fuel economy is not sufficiently developed in a low load operation region of an internal combustion engine which performs non-throttle operation, because of the lowering of an effective compression ratio with too early closing of an intake valve.

The intake valve is closed in a partial load operation region during an intake stroke and lower limit values different depending on operation regions are provided for the effective compression ratio. The intake valve is opened works at the timing of closing the intake valve showing the lower limit value for the effective compression ratio and the intake valve and an exhaust valve are opened and closed, respectively, before a top dead point of an exhaust stroke. The too much lowering of the effective compression ratio in the low load operation region is avoided and cylinder temperature and pressure at an ignition timing is sufficiently increased, to avoid the lowering of combustion efficiency and maximize the improved fuel economy effect of non-throttle operation.