To prevent dust in the atmosphere from entering a negative pressure part and adhering thereto when the negative pressure valve of a check valve is opened as a result of the internal part of a fuel tank being brought into a negative pressure.

When a negative pressure in a fuel tank is increases to a valve higher than a constant value, a negative pressure valve 18 is moved as shown in a drawing against a spring 19 and opened. The atmosphere enters, as shown in a drawing, a first atmosphere introduction chamber 22 through an atmosphere introduction port 20 for dispersion. The atmosphere is throttled by a communication hole 21 and enters a second atmosphere introduction chamber 22 for dispersion. The atmosphere flows through the periphery of the opened negative pressure valve 18 and flows to the right in a Fig. to control an internal pressure in a fuel tank. When dust entering togetherwith the atmosphere enters the two atmosphere introduction chambers 22 and 23, it is dispersed and hardly adhered to the negative pressure valve part. Reliability on the check valve is improved.