The present invention relates to a heat exchanger which can suitably regulate the quantity, the feeding position or the feeding order of heat exchange medium fed into tubes to adjust heat exchange performance according to cooling and heating load. The heat exchanger comprises a plurality of tubes placed at least one header, each tube having both ends fixed to the header, medium-distributing means installed at the header for feeding heat exchange medium to the specific tubes, a tank placed over the medium-distributing means, the tank having a medium-inlet pipe, a medium-outlet pipe and distribution passages for feeding heat exchange medium to specific regions of the medium-distributing means, and medium-regulating means installed at the tank and operated in response to a control signal for adjusting the feed of the heat exchange medium.