PURPOSE: To operate a blower automatically so that the carbon dioxide concentration in a car cabin rises.

CONSTITUTION: A thermistor thermometer 7 is mounted on a nose part as a respiration device for a driver/passenger D in a car cabin 2, and the sensing voltage Vs of the thermometer 7 is fed to a control circuit 8, which gives a drive signal to a blower 6 for ventilation of the cabin 2. The control circuit 8 calculates the respiration amount S on the basis of the sensed voltage Vs. When the carbon dioxide concentration rises owing to generation of the condition that inside of the cabin 2 is shut tightly, the respiration amount S of the driver/passenger D increases, and the blower 6 is operated if the respiration amount S reaches the operation starting level, and then the operation of the blower 6 is stopped when the ventilating causes the respiration amount S to reach the operation stopping level, so that the blower 6 is driven only when necessary to enable putting inside of the cabin 2 in a comfortable condition, which can be established with a simple configuration.