A heat exchanger provided with tubes 210 through which a medium flows and tanks 300 to which the ends of the tubes are connected to perform heat exchange of the medium with heat conducted to the tubes, wherein the tanks each provided with an end plate 320 having holes 321 for connection of the ends of the tubes and a tank plate 310 to which the end plate is fitted; and when it is assumed that a cross section of the tank in its longitudinal direction has a passage area X and a total wetted perimeter length Y for the medium, they have a relationship of XnullanullY2/4null and 0.9nulla