PURPOSE: To prevent a phenomenon where the volume of air flowing into a heat exchanger decreases due to frosting by providing a frosting groove made at the lower part of the air in flow side, and a side staging formed on the side face of a heat exchanger fin to form a side face air flow channel.

CONSTITUTION: A frosting groove 102 for preventing frosting on the upper side of a heat exchanger fin 101 through concentrated frosting is made at the lower part of each heat exchanger fin 101 located on the air flow-in side. A side staging 103 for forming a side face channel when an air inlet is interrupted due to frosting at the lower part is formed on the side face of the heat exchanger fin 101. Consequently, the air inlet between the heat exchanger fins 101 is prevented from being interrupted due to frosting entirely at the lower part of the heat exchanger fin 101. Even if the air inlet between the heat exchanger fins 101 is interrupted, heat exchanging performance is prevented from deteriorating due to reduction of each air volume.