To provide a defrosting control method for an air conditioner capable of increasing low temperature heating capacity and comfortableness by increasing heating operation time since defrosting can be performed in an optimum frosted state by preventing "idle defrosting" even in heating operation under low atmospheric temperatures.

When defrosting by mask time is continuously performed multiple times when the lowering of the temperature of an indoor heat exchanger is below a specified value, an atmospheric temperature is determined to be lower than a target temperature. Defrosting operation conditions are set such that a difference between the temperature of the indoor heat exchanger and the indoor temperature is set smaller by a specified value than an initial set value, the mask time is set longer by a specified value than an initial set value, or defrosting is not performed until the temperature of the indoor heat exchanger for each specified time lowers to a specified value. The operating mode is switched to either of these low atmospheric temperature operating conditions for the defrosting operation.