A control apparatus for a compression autoignition engine includes an engine, a state quantity setting device, a spark plug, and a controller. After the spark plug ignites air-fuel mixture to start combustion, unburned air-fuel mixture is combusted by autoignition. The controller changes, according to an operation state of the engine, a heat amount ratio that represents an index associated with a ratio of an amount of heat generated by air-fuel mixture being combusted by flame propagation, to a total amount of heat generated by air-fuel mixture in the combustion chamber being combusted.