A state detection system for an internal combustion engine includes: a memory configured to store mapping data, the mapping data being data that defines a detection mapping, the detection mapping being a mapping between an input and an output, the input being a first waveform variable and a second waveform variable and the output being a value of a combustion state variable, and the detection mapping including a joint operation of the first waveform variable and the second waveform variable based on a parameter learned by machine learning; and a processor configured to execute an acquisition process and a determination process. The acquisition process acquires a value of the first waveform variable. The determination process determines whether or not the internal combustion engine is in a predetermined operating state.