To provide an internal combustion engine and an operating method thereof, surely restraining deterioration of exhaust emission in switching fuel burnt in a combustion chamber between reformed gas and normal fuel.

This internal combustion engine 1 includes: an normal fuel injection valve 160 for injecting hydrocarbon fuel to an intake port; a reforming device 20 for reforming air mixture of hydrocarbon fuel and air to generate reformed gas containing fuel component; a preceding catalyst device 9a and a subsequent catalyst device 9b for purifying the exhaust gas from each combustion chamber 3; and an ECU 30. The ECU 30 permits switching fuel to be burnt in each combustion chamber 3 from reformed gas to normal fuel at the stage of determining that purification of exhaust emission can be favorably performed in the preceding catalyst device 9a.