To provide an engine selectively supplying high concentration oxygen air in which oxygen concentration in intake air flowing into an engine combustion chamber exceeds oxygen concentration of ordinary air.

In an engine not having an exclusive decompression machine in a high concentration oxygen air take out opening side of an oxygen enrichment membrane device, two intake air system of a first air supply system which is an atmospheric air intake passage 02 from ordinary atmospheric air (oxygen concentration 21%) and a second air supply system of a system independent from the first air supply system including a high concentration oxygen air passage 33 from the oxygen enrichment membrane device 31, the high concentration oxygen air take out side of the oxygen enrichment membrane device 31 is connected to a decompression generation part generated by engine rotation to generate high concentration oxygen air, and throttle valves 03a, 03b which are regulating valves regulating intake air flow quantity to the first air supply system and the second air supply system are provided, and an engine control unit ECU performs adjustment.