An operating method for a room air conditioner(RAC) which is capable of enhancing an alertness level of a user based upon a human electroencephalogram(EEG), includes inputting an alertness operation mode key when the user desires to heighten an alertness degree of the user, performing an air conditioning operation to maintain a first standard temperature and a range of a temperature variation corresponding thereto to enhance the user's alertness degree, performing an air conditioning operation to maintain a second standard temperature and the range of the temperature variation corresponding thereto when the user feels cold at the first standard temperature, and to maintain a third standard temperature and the range of the temperature variation corresponding thereto when the user feels hot at the first standard temperature, repeating the second step when the user feels cold at the third standard temperature, and when the user feels hot at the second standard temperature, and performing an air conditioning operation according to a general mode when the user feels hot at the third standard temperature, and when the user feels cold at the second standard temperature.