PURPOSE: To improve dehumidification and the combustion efficiency of an engine at the time of low load by providing a passing air quantity adjusting member for adjusting quantity of air passing through a condenser and a radiator.

CONSTITUTION: At the time of a low load, a motor 7c is driven to withdraw and spread the screen portion 7a of a roll-up screen 7 rolled round a roll-up portion 7a to shield the front of a condenser 2 partially. As a result, one part of cooling air is shut off to lower the cooling efficiency of a condenser 2 and a radiator 6. Liquified catalyst quantity supplied to an evaporator side is thereby reduced to raise a temperature at the outlet of the condenser 2. According to it, discharge pressure is increased so that the rotation of a control member from a partial working position to a full working position becomes quicker. Consequently, the dehumidity of a cooling unit at the time of low load is improved. The temperature of cooling water supplied to an engine is not lowered so much due to the lowering of the cooling efficiency so that the combustion efficiency of the engine is improved.