A prior art device disclosed, for instance, in Japanese Patent Laid-Open No. 2005-248909 detects the combustion status of an internal combustion engine in accordance with the rotational fluctuation of the internal combustion engine. The rotation speed of the internal combustion engine changes in accordance with a combustion cycle. In a low rotation speed region where the engine speed is lower than a predetermined speed, this device detects the combustion status by using the rotational fluctuation amount of a first frequency component synchronized with the combustion cycle of the internal combustion engine. In a high rotation speed region, on the other hand, this device detects the combustion status by using the rotational fluctuation amount of a second frequency component, which is generated, for instance, from the torsion of a crankshaft and higher in frequency than the first frequency component. Therefore, this device detects the combustion status in a wide rotation speed region.
Patent Document 1:    Japanese Patent Laid-Open No. 2005-248909
Patent Document 2:    Japanese Patent Laid-Open No. Hei11-22504
Patent Document 3:    Japanese Patent Laid-Open No. Hei3-294636
Patent Document 4:    Japanese Patent Laid-Open No. Sho63-47544