35 executes the second operation after the ignition of the air fuel mixture i e at the start of the second half of the flame propagation control device 35 outputs an em wave driving signal as the second operation em wave emitting device 13 repeatedly outputs microwave pulses from radiating antenna 16 when the em wave driving signal is received microwave pulses are emitted repetitively throughout the second half of the flame propagation the microwave pulses resonate in each receiving antenna 52 in the area close to the outer circumference of combustion chamber 20 where the two receiving antennas 52 are located an intense electric field is formed during the second half of the flame propagation the propagation speed of the flame increases due to absorption of the microwave radiation when the flame passes the intense electric field in this embodiment an intense electric field is formed close to the outer circumference of combustion chamber 20 during flame propagation this allows for an increase in the propagation speed of the flame close to the outer circumference of combustion chamber 20 in the first modification em wave emitting device 13 is provided such that plasma is generated by microwave radiation emitted from radiation antenna 16 the energy per unit time of the microwave radiation from em wave generating device 31 is.