To provide a means for improving ignitability in the case in which low cetane number fuel to be hardly ignited is used, in a premixture compression ignition engine for compressing and igniting air-fuel premixture.

In a premixture compression ignition engine, the timing is synchronized with the suction of a premixture 8 by releasing of an intake valve 6, ozone generated by an ozonizer 9 is mixed in the premixture 8 at a part near the intake valve 6, and therefore, ozone added mixture having the high ozone concentration is formed in high-temperature mixture in a cylinder in the termination of the compression stroke. Therefore, the ignitability of low cetane number fuel is improved by chemical activity of ozone.