To provide a ventilation system carrying out appropriate and efficient ventilation in response to an indoor state by connecting a sensor unit detecting gas concentration.

The ventilation system is equipped with: a sensor signal input part 3 connecting the sensor unit 1 turning a contact output ON when the gas concentration is high and turning the contact output OFF when the gas concentration is low, and inputting a contact output signal of the connected sensor unit 1; a blower 4 for carrying out ventilation; a blower driving part 5 varying an air capacity of the blower 4 three levels or more; and an operation part 6 setting an operation mode of automatic operation/continuous operation/stopping of the blower 4. In the automatic operation mode, if the contact output of the sensor unit 1 is OFF, the blower 4 is operated at a slow operation, if the contact output of the sensor unit 1 is ON, the blower 4 is operated for a predetermined time at a high speed operation, and if the contact unit of the sensor unit 1 is still ON thereafter, the air capacity of the blower 4 is raised, and it is operated in particularly high operation until the contact output of the sensor unit 1 becomes OFF.