Abstract: A heat exchanger, in which flowing medium flows, includes a first tank, a second tank, a core part and a flow accelerating means. The first tank is provided with an inlet port, the second tank being disposed apart from the first tank. The core part has a plurality of tubes and a plurality of fins. The tubes have both end portions being fluidically connected with the first tank and the second tank, respectively. Each of the fins is arranged between the adjacent tubes. The flow accelerating mean is provided inside the first tank so as to accelerate a flow speed of the flowing medium, which enters an inner space of the first tank through the inlet port, in the first tank in a longitudinal direction of the first tank.