To provide an apparatus and a system for arbitrarily adjusting the amount of exhaust without increasing the size of a ventilation apparatus, when it is necessary to install specified space requiring a large amount of venting in a newly constructed building, such as a house.

A backflow preventer in which an exhaust check valve is provided inside separately from a main ventilator, and an auxiliary ventilator are provided for a specified space exhaust circuit as a means of an exhaust system in a separated state from another space.