The present invention relates to a heat exchanger, in which the flow of a heat exchange medium flowing through tubes is selectively controlled, and opened and closed in order to control heat exchange capability according to cooling and heating loads. More specifically, the invention relates to a heat exchanger, in which one distribution hole is constructed for one tube, so that temperature can be minutely controlled with small temperature deviation in each step, and the opening and closing method of the distribution hole is configured in a sliding type that uses a slide valve, so that the shapes of a header and a tank are simplified, and a clamping operation is also improved.