PURPOSE: To highly precisely detect a misfire with a simple structure by mounting a vibration detecting means on an internal combustion engine, and detecting the misfire on the basis of the detected vibration.

CONSTITUTION: During the operation of an internal combustion engine m1, a vibration detecting means M2 detects the vibration of the engine M1, and an ignition timing detecting means M3 detects the ignition timing based on the rotation of the engine M1. A combusting state signal arithmetic means M4 processes the vibration signal detected by the means M2 synchronously with the ignition timing detected by the ignition timing means M3 and operates the combusting state signal instructing the combusting state of the engine M1. A misfire judging means M5 judges the misfire when the signal computed by the combusting means M4 does not satisfy a standard value. Thus, the misfire can be highly precisely detected with a simple structure.