PURPOSE: To prevent the occurrence of sudden fluctuation in engine output torque during switching of a compression ratio, by a method wherein a variable compression ratio device is forced to perform supercharging when it is actuated on the low compression ratio side, meanwhile, it is forced to stop supercharging or widely reduce supercharging when it is actuated on the high compression ratio side.
CONSTITUTION: An eccentric bearing 6 is rotatably located between a piston insertion hole 5, formed in the end part of a connecting rod 3 through which a crank shaft and a piston 2 are interconnected, a piston pin 4. Through engagement of a lock pin 8 with a lock pin engaging hole 9 formed in the eccentric bearing 6, the piston 2 is displaced to a high and a low stage, and a compression ratio is formed so that it is switchable in 2 stages of the one being a high and the other being a low stage. In this case, when a low compression ratio is selected, a vacuum switching valve 47 is opened by means of an ECU 23 and an actuator 40 is driven to close a waist gate valve situated to a turbocharger 31 to perform turbocharging. Meanwhile, when a high compression ratio is selected, control is effected so that, reversely, supercharging is stopped or reduced.