PURPOSE: To avoid increase in the capacity of a memory by performing the ignition timing control in the event of failure in a knock sensor system at the base ignition timing according to a two-dimensional map set at a knock avoid point while in the knock range.

CONSTITUTION: A control means 16 prohibits knock control when No. 1 judging means 13 judges that the signaling line of a knock sensor 12 is troubled, and an ignition means 17 is driven at the ignition timing according to a two-dimen sional map read out of a memory 15. If judgement made by No.2 judging means 14 is such that the operating condition of an engine 11 lies in the off-knocking range, the ignition means 17 is driven at the ignition timing which gives max. torque. In the knocking range, on the other hand, the ignition means 17 is driven at the base ignition timing according to the two-dimensional map, at which the ignition timing is set at the knock avoid point. This requires use of only one two-dimensional map, and proper ignition timing can be obtained even in the event of failure in the knock sensor signaling line without increasing the memory capacity.