A heat exchanger capable of reducing the size of a header pipe, having a sufficiently high strength with respect to a pressure and provided with caps of a structure in which imperfect cleaning does not occur during a cleaning step, wherein the caps which seal both end portions of a cylindrical section constituting the header pipe are provided with annular grooves in which circumferential edges of the end portions of the cylindrical section are engaged along the whole circumferences thereof, whereby the cylindrical section is fitted in inner portions of the caps, this enabling an axial size of the header pipe to be reduced, the pressure resistance of the caps being improved by forming large-thickness parts on the sealed portions of the caps, outer flat portions of the caps being provided with projecting parts.