Ventilator including a case having a first flow passage and a second flow passage therein, at least one fan in the case for supplying room air to a room through the first flow passage, and guiding outdoor air to the room through the second flow passage, and a heat exchanger in the case, the heat exchanger having a working fluid therein for repeating evaporation (heat absorption) and condensing (heat dissipation) by a temperature difference of the air flowing through the first flow passage and the second flow passage, for making the air flowing through the first flow passage and the air flowing through the second flow passage to heat exchange, thereby providing a ventilator which is thin, can be installed conveniently, and can recover a thermal energy from room air, and reduce a pressure loss and noise.