An air conditioner comprising: an indoor unit; an outdoor unit configured to detect an outdoor air temperature; and a control device mounted on the indoor unit and configured to perform operation controls of the indoor unit and a heater placed indoor, wherein the control device comprises: a storage unit configured to store therein threshold data indicating a first threshold and a second threshold that are criteria used for determining whether to enable or disable control of the heater; a heater control determination unit configured to read the threshold data from the storage unit, to enable a control of the heater when the outdoor air temperature is equal to or lower than the first threshold, and to disable the control of the heater when the outdoor air temperature is equal to or higher than the second threshold; a heater operation control unit configured to set an operation of the heater to be always-off when the control of the heater is disabled, and to ON/OFF control the operation of the heater based on a relation between an indoor temperature and a set temperature when the control of the heater is enabled; a reception unit configured to receive a threshold designation signal input by a user and designating at least one of the first threshold and the second threshold; and a threshold setting unit configured to set, based on designation by the received threshold designation signal, the threshold data stored in the storage unit wherein the first threshold is lower than the second threshold. an indoor unit; an outdoor unit configured to detect an outdoor air temperature; and a control device mounted on the indoor unit and configured to perform operation controls of the indoor unit and a heater placed indoor, wherein the control device comprises: a storage unit configured to store therein threshold data indicating a first threshold and a second threshold that are criteria used for determining whether to enable or disable control of the heater; a heater control determination unit configured to read the threshold data from the storage unit, to enable a control of the heater when the outdoor air temperature is equal to or lower than the first threshold, and to disable the control of the heater when the outdoor air temperature is equal to or higher than the second threshold; a heater operation control unit configured to set an operation of the heater to be always-off when the control of the heater is disabled, and to ON/OFF control the operation of the heater based on a relation between an indoor temperature and a set temperature when the control of the heater is enabled; a reception unit configured to receive a threshold designation signal input by a user and designating at least one of the first threshold and the second threshold; and a threshold setting unit configured to set, based on designation by the received threshold designation signal, the threshold data stored in the storage unit wherein the first threshold is lower than the second threshold. a storage unit configured to store therein threshold data indicating a first threshold and a second threshold that are criteria used for determining whether to enable or disable control of the heater; a heater control determination unit configured to read the threshold data from the storage unit, to enable a control of the heater when the outdoor air temperature is equal to or lower than the first threshold, and to disable the control of the heater when the outdoor air temperature is equal to or higher than the second threshold; a heater operation control unit configured to set an operation of the heater to be always-off when the control of the heater is disabled, and to ON/OFF control the operation of the heater based on a relation between an indoor temperature and a set temperature when the control of the heater is enabled; a reception unit configured to receive a threshold designation signal input by a user and designating at least one of the first threshold and the second threshold; and a threshold setting unit configured to set, based on designation by the received threshold designation signal, the threshold data stored in the storage unit wherein the first threshold is lower than the second threshold.