A corrugated fin heat exchanger is provided in which the direction in which louvers are cut and raised is inclined in one direction only, and in which heat transfer performance is improved above that of conventional fins. To accomplish this, the relationship H>Qup/(Qup−1)×ΔH is satisfied.H represents the core height of the heat exchanger,Qup represents the ratio of the amount of heat exchanged per corrugation between one-directional louver fins and multi-directional louver fins in an airflow part,and ΔH represents the amount of increase in a heat transfer reduction region of a heat exchanger core as a result of changing from multi-directional louver fins to one-directional louver fins.