PURPOSE: To enhance switching responsiveness by mounting a throttle valve for throttling oil flow which is branched to a lubricating oil passage from the discharge side passage of an oil pump, and installing a means for controlling the throttle valve for the complete operation of a cam switching mechanism.

CONSTITUTION: A four-cylinder engine is provided with a cam switching mechanism 11 which switches a cam for opening and closing an air intake valve, and a cam switching mechanism 12 which switches a cam for opening and closing an exhaust valve for respective cylinders. A pressure oil sage 13 which leads driving pressure oil to the respective cam switching mechanisms 11, 12 and a lubricating oil passage 16 are disposed, branched from the discharge side passage 15 of an oil pump 14. Moreover, a throttle valve 50 for throttling oil flow which is shunted to the lubricating oil passage 16 is mounted. In addition, a controller 19 is installed as a means for controlling the throttle valve 50 so that it may throttle the lubricating oil passage 16 temporarily at the time of operating the respective cam switching mechanisms 11, 12. As a result, oil pressure which is led to the cam switching mechanism is increased, so it is possible to operate the cam switching mechanism completely and enhance switching responsiveness.