1. A method for controlling a refrigerating system which includes an evaporator for evaporating refrigerant by reducing pressure of the refrigerant supplied into the evaporator and electric-driven means for controlling a flow rate of the refrigerant supplied into the evaporator, comprising the steps of: obtaining a first signal representing an evaporating temperature of the refrigerant; obtaining a second signal representing a superheated temperature of the refrigerant at an outlet of the evaporator; processing the second signal by a phase lag element to obtain a processed signal; obtaining a difference signal representing a difference between the processed signal and the first signal; and controlling a flow rate of the refrigerant in accordance with the difference signal.