To extend a period of time until a clogged state caused by frosting occurs between the fins by a method wherein a frosting inducing section having no fins there is formed at a linear line part of a tube positioned at an air inlet part of a heat exchanger.

A heat exchanger 1 is provided with an inlet tube 3a at one end of a tube. A U-shaped section 5a, a linear line section 7a, a U-shaped section 5b and a linear line section 7b are repeatedly connected in sequence to the inlet tube 3a. A U-shaped section 5i is bent at a linear line section 7h located at an air inlet side A of the heat exchanger 1. Then, a fin pitch p1 between the fins 11 of the linear line section 7g near the air inlet side A where frosting is apt to occur is set to be larger than a fin pitch p2 between the fins 11 of the linear line section near an air outlet side B and at the same time a frosting inducing section 7h not provided with fins is arranged at the linear line section 7h located at the air inlet side A rather than at the linear line section 7g. With such an arrangement as above, it is possible to delay a timing of defrosting operation.