There is provides a heat exchanger comprising: a plurality of tubes (110) stacked on each other; and a pair of header tanks (140), each header tank (140) having a flow section (151) in which fluid flows, extending in a direction of stack of the tubes (110), wherein both end sections (111) of the tubes (110) in the longitudinal direction are joined to the pair of header tanks (140), the flow section (151) of each header tank (140) and the inside of each tube (110) are communicated with each other, a tip position (a) of the tube end section (111) is arranged in an outside region of the flow section (151), and an inner wall width size (b) of the flow section (151) is smaller than a size (c) in the width direction of the header tank (140) at the tube end section (111).