An air conditioner, comprising: a plurality of indoor units configured to perform an air conditioning operation; and an outdoor unit driven by a control program and configured to drive the indoor units based on the control program, wherein the outdoor unit includes a mode conversion unit configured to select a mode between a driving control mode for driving the outdoor unit and the indoor units by execution of the control program, and an upgrade mode for upgrading the control program, wherein the mode conversion unit is configured to determine an error of an upgrade signal, and to maintain the outdoor unit in the driving control mode in the occurrence of an error of the upgrade signal; wherein the outdoor unit is connectable to a central control server through a network; characterized in that the outdoor unit further includes: a communication unit configured to receive the upgrade signal and an upgrade control program from the central control server; a storage unit configured to store the control program; and a controller configured to execute the control program according to the driving control mode, or to delete the control program and to store the upgrade control program in the storage unit according to the upgrade mode, wherein the mode conversion unit is configured to check a mode to be executed by the outdoor unit once power has been supplied to the outdoor unit, and to set the outdoor unit to the upgrade mode when the upgrade signal has been received at the time of an initial driving or re-driving of the outdoor unit, and to set the outdoor unit to the driving control mode when no upgrade signal has been received. a plurality of indoor units configured to perform an air conditioning operation; and an outdoor unit driven by a control program and configured to drive the indoor units based on the control program, wherein the outdoor unit includes a mode conversion unit configured to select a mode between a driving control mode for driving the outdoor unit and the indoor units by execution of the control program, and an upgrade mode for upgrading the control program, wherein the mode conversion unit is configured to determine an error of an upgrade signal, and to maintain the outdoor unit in the driving control mode in the occurrence of an error of the upgrade signal; wherein the outdoor unit is connectable to a central control server through a network; characterized in that a communication unit configured to receive the upgrade signal and an upgrade control program from the central control server; a storage unit configured to store the control program; and a controller configured to execute the control program according to the driving control mode, or to delete the control program and to store the upgrade control program in the storage unit according to the upgrade mode, wherein the mode conversion unit is configured to check a mode to be executed by the outdoor unit once power has been supplied to the outdoor unit, and to set the outdoor unit to the upgrade mode when the upgrade signal has been received at the time of an initial driving or re-driving of the outdoor unit, and to set the outdoor unit to the driving control mode when no upgrade signal has been received.