A ventilation system and a pressure intensifying apparatus can smoothly supply air to an indoor room by increasing required air volume and external static pressure. The ventilation system includes: a duct branched into at least rooms; a ventilator connected to the duct, for selectively or simultaneously exhausting indoor air and supplying outdoor air; and a pressure intensifying apparatus including a case in which air supply unit/air exhaust unit connected to the duct are formed, and a fan for increasing static pressure of air supplied/exhausted through the duct.