The fuel tank unit includes a fuel cutoff valve fastened to a support body. The support body and the casing of the fuel cutoff valve are made of first and second resin materials having different rates of fuel swelling. The fuel cutoff valve is fastened to the support body via a fastening mechanism. The fastening mechanism entails engaging a first support engaging portion and a second support engaging portion of the support body in first and second fastening engaging portions composed of hooks formed on the casing of the fuel cutoff valve, at first and second hook engagement levels that represent the bite of the hooks. The engaging portions are designed such that prior to fuel swelling of the support body and the base cover first hook engagement exceeds second hook engagement, whereas subsequent to fuel swelling second hook engagement exceeds first hook engagement.