To properly ventilate while always maintaining an air in a room at a high oxygen concentration.

In this indoor ventilation system, the amount of ventilation air per hour is at least 1/2 of an indoor volume based on the Building Standard Law. An oxygen enrichment unit 9 formed of an oxygen enrichment membrane 18, a vacuum pump 15 and a control unit 20 is installed on the air supply side, and the oxygen-enriched air discharged from the oxygen enrichment unit 9 is taken into the room R. The discharged amount of the air is 1/16 or more of the total amount of the led outside air corresponding to the ventilated air amount.