PURPOSE: To enhance the sensing performance for the ion current even under the operating conditions of high speed, high load, etc., by shortening the ignition discharge time forcedly, and providing sensibility for the masked ion current within the discharge time.

CONSTITUTION: A power transistor 1 is controlled in synchronization with the revolving speed of an internal combustion engine, and commencement of current feeding and shutting thereof to an ignition coil are performed. A voltage is impressed to the ignition plug 5 of each cylinder of the internal combustion engine, and the ion current due to combustion is sensed by an ion current sensing device 7, and an appropriate sensing signal is emitted. During the ignition discharge, the current feed is started forcedly to stop the discharge, and then the ion current in the ignition plug 5 is sensed. Reigniting is made after a certain period of time or after sensing of ion current.