PURPOSE: To maintain the performance against the temperature drift, make a device lightweight and small-sized, and reduce the cost by using a mixed liquid of ethylene glycol and diethylene glycol as a cooling liquid filled in a liquid-tight space.

CONSTITUTION: A liquid-tight space 9 is formed with a projection tube 1, a spacer 2, a radiator 3 and a lens 4, and a cooling liquid 9a is filled to cool the projection tube 1. A mixed liquid of ethylene glycol and diethylene glycol is used as the cooling liquid 9a. This cooling liquid has little permeation loss quantity, the vapor pressure is low, the inner pressure of the space 9 is low, thus the cooling liquid 9a can be brought into direct contact with the lens 4. The deterioration in lens performance due to the temperature drift dose not occur, a liquid with the refraction factor approximate to that of the lens 4 can be generated, the contrast is improved. The device is thereby made lightweight and small-sized, and the cost can be reduced.