PURPOSE: A method of a sealing test is provided to check a sealing state of an optical system used in a projection television within a short time. CONSTITUTION: A cooling portion(10) is installed on a working plate(20). A nozzle(1) and a cylinder(30) are arranged on an upper portion of the cooling portion(10). A switch and an instrument panel(40) are installed on the working plate(20). In a method of a sealing test, the first sealing test is performed by checking a variation of vacuum pressure within the cooling portion(10) after the vacuum pressure is applied to an empty inner space of the cooling portion(10). The second sealing test is performed by checking the variation of vacuum pressure within the cooling portion(10) after the first sealing test is performed and the supply of the vacuum pressure is stopped. The third sealing test is performed by checking the variation of vacuum pressure within the cooling portion(10) after the second sealing test is performed.
