Abstract: An electric signal T1 representing the evaporating temperature of refrigerant is processed by a phase lead network and thus converted to T1' with a single time constant so as to compensate for the time lag due to the time constant of a temperature sensor for sensing the evaporating temperature. An electrical signal T2 representing superheated vapor temperature at the outlet of an evaporator is processed by a phase lag network and thus converted to T2', so as to have a larger time constant than that of the unprocessed signal T2 and a signal corresponding to .DELTA.T'=T1'-T2' is used to control the degree of valve opening of an electric expansion valve.