PURPOSE: To reduce the cost and improve the fuel consumption rate by providing pump mechanism for making pressure act upon an oxygen enriched air producing mechanism so as to produce oxygen enriched air, additionally with a pump driving mechanism driven by the intake negative pressure of an internal combustion engine as a drive source.

CONSTITUTION: An oxygen enriched air supply device for supplying oxygen enriched air into an internal combustion engine 2 is provided with an oxygen enriched air producing mechanism 22 for producing oxygen enriched air, a pump mechanism 24 for making inlet pressure act upon the mechanism 22 to produce oxygen enriched air, a storage tank 26 for storing produced oxygen enriched air, and valve mechanism 28 for releasing oxygen enriched air in the storage tank 26 when the opening of an intake throttle valve 8 becomes more than the specified value. In this case, the pump mechanism 24 is integrally provided with pump driving mechanism 60 driven by the intake negative pressure of the internal combustion engine 2 as drive source. That is, a pressure chamber 62 is partitioned on one side of a diaphragm 34 for partitioning a pump chamber 36 on the other side in a body 32, and this pressure chamber 62 is provided with a pressing spring 64 and communicated with an intake passage 6 through a pressure lead passage 66.