To predict generation of surge in turbofreezer accurately without increasing the machine and an equipment cost by using a generally provided measuring instrument.

A differential pressure μ0 is obtained from an outlet pressure 7a and an inlet pressure 8a of the turbofreezer. A converted wind volume Φ' is calculated with the inlet pressure 8a, an inlet temperature 10a, an electric motor current 11a, and the differential pressure μ0 of the turbofreezer. A serge curve S of the turbofreezer is determined based on the relationship between the converted wind volume Φ' and the differential pressure μ0. By inputting the calculated converted wind volume Φ' for the surge curve S, a surge generating differential pressure X at a given converted wind volume Φ' is obtained. The generation of surge is predicted by comparing the surge generating differential pressure X and the differential pressure μ0.