A rotation angle sensor detects the rotation speed of a crankshaft of an engine. An ECU extracts a first frequency component synchronized with combustion cycles of the engine that is contained in a rotation speed signal obtained by the detection of the rotation angle sensor, and a second frequency component having a higher frequency than the first frequency component. It detects a combustion state by using the rotation variation amount of the first frequency component in a lower rotation range than a specified engine speed, and by using the rotation variation amount of the second frequency component in a higher rotation range.