PURPOSE: To simplify a valve device as a whole, by providing a first valve device for an openable valve in a primary passage of a vacuum system and a second valve device for a check valve in a secondary passage.

CONSTITUTION: When a degree of vacuum in a secondary passage 1 is greater than that in a primary passage 2, a valve member 4 of a first valve device 3 is closed, but the air leaks little by little through a small aperture 6. As a result, when the degree of vacuum in the primary passage 2 becomes larger than that in the secondary passage 1, the valve member 4 is opened, and a valve member 8 of a second valve device 7 is closed. Accordingly, the valve operation may be achieved automatically by a pressure differential between the primary passage and the secondary passage with a simple structure.