Abstract: A controller for an air conditioner is provided wherein an operation mode is not unnecessarily changed in accordance with variations in temperature. When room temperature t is in a range of T+3.0>t.gtoreq.T+1.5, and when a timer expires (steps 212, 214), it is determined whether the room temperature has a tendency to fall (step 218). When the room temperature has a tendency to fall, it is predicted that the room temperature will be in an observing region although the operation mode is not changed. For this reason, the current operation mode is kept. On the other hand, when the room temperature does not have a tendency to fall, a cooling mode is set (step 210). Further, when the room temperature t is in a range of T-3.0<t.ltoreq.T-1.5, and when the timer expires (steps 222, 224), it is determined whether the room temperature has a tendency to rise (step 228).