To provide a constitution of an exhaust gas sealed hydrogen engine, in particular, for directly recycling high temperature steam exhausted from a hydrogen engine, then circulating it to an original engine, while aiming at a system for not wholly discharging exhaust gas from the engine such as carbon dioxide or the like that destroys environment.

This automobile-mounted exhaust gas sealed circulating hydrogen engine has a means for recycling high temperature steam gas exhausted from the hydrogen engine for directly decomposing it into hydrogen and oxygen, and is characterized by connecting, circulating and supplying the generated hydrogen and oxygen to the original hydrogen engine by generating the hydrogen and the oxygen.