To locate a float valve in a predetermined position within a case in a state that the contact position with the case is reduced as much as possible.

A valve for a fuel tank is provided with the case 1 and the float valve 2. The case 1 attached to the fuel tank T is provided with a passage L for gas such as fuel vapor gas connecting the interior to the exterior of the fuel tank T. Then the float valve housed within the case 1 is moved to the valve-closing position for closing the passage L by the ascent of fuel within the fuel tank T or by the inclination caused to a vehicle. The valve for the fuel tank has a guiding means 4 for guiding the movement of the float valve 2, and the clearance S is always formed in each position around the float valve 2 between the side 2b of the float valve 2 and the inner surface 1d of the case 1 by the guiding means 4.