A working fluid containing chlorotetrafluoroethane and at least two fluorinated hydrocarbons having a boiling point of not higher than -40.degree. C. under atmospheric pressure selected from the group consisting of methane derivatives and ethane derivatives which consist of one or two carbon atoms, hydrogen atoms and fluorine atoms, which has very small influence on the ozone layer in the stratosphere and decreased flammability and is suitable as a substitute working fluid for chlorodifluoromethane.