PURPOSE: To stabilize the combustion over an extensive range and promote maintenance free for a spark plug by performing each ignition by means of a high heat value plug at the high load side of an engine and by means of a low heat value plug at the low load side, respectively.

CONSTITUTION: In a variable cycle engine which is formed so as to make supply air being discharged out of a supercharger 6 conductible into a cylinder 9, and so as to perform one time combustion while a crankshaft 17 rotates one time when an engine is shifted to the high load side, and when it is shifted to low load side, to perform one time combustion while the crankshaft 17 rotates plural times, a high heat value plug 7 and a low heat value plug 8 both are installed in a cylinder head 1 in advance. Each ignition is performed by the plug 7 at the high load side of the engine and by the plug 8 at the low load side, respectively. With this constitution, combustion is stabilized over an extent range from the low load to the high load, while maintenance free for these spark plugs 7, 8 is well promotable.