A ventilation system includes a connection determinator configured to determine that N-th, second, and sixth humidity controlling ventilation devices are connected to an identical second duct bank in a situation where when fans of the second and sixth humidity controlling ventilation devices are driven, and the rotational speed of a fan of the N-th humidity controlling ventilation device is changed, a power detector detects fluctuations in the power consumed by the fan of each of the second and sixth humidity controlling ventilation devices.