PURPOSE: To make full use of an engine capacity and perform efficient automobile traveling by equipping a low-speed revolution spark plug and a high-speed revolution spark plug in a combustion chamber, and selectively making either spark plug discharge in response to the operating condition of the engine.

CONSTITUTION: Two kinds of spark plugs, a low-speed revolution spark plug 5a and a high-speed revolution spark plug 5b, are provided in each cylinder 6 and voltage is impressed on one of the two kinds of spark plugs 5a, 5b by means of a plug changeover switch 8 provided between a distributor 4 and the spark plugs 5a, 5b. Based on a signal input from an engine speed sensor 21, a control device 20 actuates the plug change-over switch so as to impress voltage on the low-speed revolution spark plug 5a, when the number of revolutions of an engine is not more than a specified speed, and to impress voltage on the high-speed revolution spark plug 5b when the number of revolutions of the engine is not less than the specified speed.