To accurately detect degradation in combustion state of an engine.

A combustion state detection device for an engine comprises: an angular velocity difference calculation means 51 for acquiring differences between a predetermined reference angular velocity and angular velocities of a crankshaft of the engine 10 at two or more of points of time detected by an angular velocity detection means 41; an integration means 54 for integrating the angular velocity differences acquired by the angular velocity difference calculation means 51 so as to acquire an integrated angular velocity difference; an integration period set means 53 for setting an integration period where the integration is performed by the integration means 54; and a combustion deterioration determination means 55 for determining that the combustion state of the engine 10 is deteriorated, when the integrated angular velocity difference integrated during the integration period set by the integration period set means 53 exceeds a predetermined combustion deterioration determination threshold value.