An ammonia burning internal combustion engine in which, in addition to ammonia, a reformed gas reformed at a reformer is fed into a combustion chamber. When a reforming ability of the reformer is less than a predetermined reforming ability, an ammonia ratio is increased to a ratio more than the ammonia ratio after completion of engine warmup set in advance in accordance with an operation state of the engine, and secondary air is fed from a secondary air feeding device into an engine exhaust passage upstream of an exhaust purification catalyst.