PURPOSE: To perform most effective operation of the air conditioner in obtaining a preset room temperature at a time of an office start by a method wherein the relationship between the air conditioner load and the air conditioner perfomance capablity is obtained base on the record of air conditioner operation state in the past and thus obtained data is referred to in calculating the next start time point of the air conditioner.

CONSTITUTION: In the start control of the air conditioner installed in a building, the time when the air conditioner starts and the time the room temperature reaches a preset value of a day in the past, for example, yesterday, are memorized. Further, a natural room temperature (t) at the time of business start is predicted based on the room temperatures t1 and t2 at the two time points T1 and T2 before the operation start of the air conditioner, and a temperature difference Δt between the predicted temperature and preset value. In this instance, two temperature differences Δt of the day and the previous day are obtained. On the base of the data, an optimum start time point Ts of the air conditioner of the day can be obtained based on a predetermined formula. The calculations are performed in a CPU 10, and when the appointed time point comes, the CPU 10 provides a start command through a process input-output unit 50 to a power switchboard 60 so as to start the operation of the air conditioner 30.