A finned heat exchanger characterized by disposing main plural raised portions 24a, 24b, 24c only on one side of the surfaces of fins 11, between heat transfer tubes 13 adjacent in the transverse direction, defining the width Wf of the raised portions 24a, 24b, 24c in the principal direction of air stream at about ⅓ of the distance Wb between adjacent raised portions in the principal direction of an air stream, and disposing grooved insulating means such as slits 31, slots 32, or raised portions 33c, 35, 37 in the portion of the fins where there is a temperature difference between fluids flowing in adjacent heat transfer tubes 13 in the principal direction of air stream, said insulating means disposed on the surface of the fins 11 near the middle of the fins between adjacent heat transfer tubes 13 in the principal direction of the air stream to suppress the conduction of heat through the fins caused by fluids flowing in adjacent heat transfer tubes in the principal direction of air stream, and to effectively enhance the heat exchange capacity and the heat transfer efficiency by the leading edge effect of temperature boundary layer of the slits 31, slots 32, or raised portions 33c, 35, 37.