An internal combustion engine 100 includes a cylinder 102 that defines a combustion chamber 196 causing a premixed gas to be combusted, a piston 120 that defines the combustion chamber 196 together with the cylinder 102, and reciprocates in the cylinder 102, and an active species generator 150 that generates active species. The internal combustion engine 100 promotes combustion of the mixed gas by the active species generated by the active species generator 150. The piston 120 includes an active species generation chamber 194 that is formed therein and open to a top surface of the piston 120, and in which the active species generator 150 generates the active species.