Abstract: The operational temperature TP is determined as a function of the compartment temperature Tr. The blower speed is controlled along the characteristic line, the slope of which varies with the operational temperature TP. When the temperature of engine cooling water reaches a predetermined temperature, the speed of the blower is increased along the slope corresponding to the compartment temperature Tr. When the operational temperature TP is at least the predetermined temperature Tl, the speed of the blower is controlled as a function of time so that any violent increase in the blower speed may be prevented.