To provide an easy method of predicting air conditioning load which has enough accuracy for operation of a heat storage tank such as a general middle scaled building, and which is very low cost.

In prediction of an air conditioning load in a heat storage air conditioning installation, a load actual result value Qn-1 at a previous day of a day to be predicted and a load prediction value Qn-1,p are estimated, and the weighted average thereof is calculated following an equation Q'n,p=αQn-1+βQn-1, p (α, β: positive weighting coefficients, and α+β=1.0) as a primary predicted value of the load, and the load actual result value Qn-1 on a previous day and the primary predicted value Q'n,p of the load are corrected with the aid of an actual result lower limit value and a prediction upper limit value obtained on the basis of the load movement average over past several days to provide a predicted value on a day to be predicted. The correction of the primary prediction value Q'n,p is ensured by multiplying the value Q'n,p by a weekday coefficient for example.