To provide an internal combustion engine and its operating method which are capable of preventing reformed fuel surely from remaining in a reforming device or the like while limiting the cost increase and a deterioration in efficiency of the use of the space.

The internal combustion engine 1 generates motive energy by combusting an air-fuel mixture of the reformed fuel and air within a combustion chamber 3. For this purpose, the internal combustion engine 1 is equipped with the reforming device 20 for reforming a mixture of a hydrocarbon-based fuel and the air to produce the reformed fuel containing CO and H2, a bypass pipe L2 for introducing the air into the reforming device 20, a reformed fuel feeding pipe L3 for introducing the reformed fuel from the reforming device 20 to each combustion chamber 3, and an ECU 30 for stopping fuel feed to the reforming device 20 when an engine halting command has been issued and then stopping ignition of the air-fuel mixture after having carried out the ignition of the air-fuel mixture in each combustion chamber 3 during a predetermined period.