The present invention provides a heat exchanger. A tank main body (7) of a header tank (3) of a condenser (1) has slits (14) formed in a circumferential wall (7a) thereof at positions between opposite end surfaces of the circumferential wall (7a) and heat exchange tubes (6) at the opposite ends. Plate-shaped closure members (8) are inserted into the slits (14) from the outside of the tank main body (7) and joined to the circumferential wall (7a) of the tank main body (7). Drain openings (18) are formed in the circumferential wall (7a) of the tank main body (7) at positions on the outer sides of the closure members (8) as viewed in the longitudinal direction of the tank main body (7). A portion of a surface of each closure member (8) which faces outward in the longitudinal direction of the tank main body (7) forms a portion of a peripheral edge of the corresponding drain opening (18).