A ventilation system includes a plurality of ventilation devices, a sensor, a setting unit, and a transmitter-receiver. The sensor is attached to a predetermined ventilation device of the plurality of ventilation devices. The sensor detects a state of air. The setting unit sets, from the plurality of ventilation devices, a first ventilation device to which the sensor is to be attached, and a second ventilation device with which a detection signal of the sensor is to be shared. The transmitter-receiver receives the detection signal of the sensor and transmits the detection signal to control devices of the first and second ventilation devices. The first and second ventilation devices are set in the setting unit. Each of the control devices of the first and second ventilation devices controls an associated one of the ventilation devices based on the detection signal transmitted from the transmitter-receiver.