An apparatus is for inhibiting fuels from flowing out of fuel tanks, and includes a cover, a breather pipe, an upper case, a lower case, and a floating valve. The upper case is welded to the breather pipe in an airtight manner so as to form a chamber for holding the lower case, in which the floating valve is accommodated, therein. Moreover, the breather pipe is welded to the cover in an airtight manner. The apparatus not only obviates O-rings but also is downsized, because the upper case and the breather pipe are sealed by welding.