A method for controlling a gas furnace including measuring an indoor temperature, comparing the indoor temperature with a set temperature for heating, and operating the gas furnace in weak heating with a predetermined heating capacity, which is lower than a maximum heating capacity of the gas furnace, when the set temperature for heating is higher than the indoor temperature by a temperature value, which is less than a predetermined temperature value. The predetermined heating capacity for weak heating is maintained at a current heating capacity for weak heating, when the gas furnace operates in weak heating, and when an operating duration with the current heating capacity for weak heating is less than a first time value.