In a cover unit for a fuel tank, a cover section is attached to the fuel tank to cover an opening of the fuel tank; an electronic-circuit containing section contains an electronic circuit; and an electric conductor is attached to penetrate the cover section and is adapted to electrically connect the electronic circuit with an electrical equipment within the fuel tank. Moreover, a vapor passage is formed between the cover section and the electronic-circuit containing section, and is adapted to discharge fuel exuded from a gap between the cover section and the electric conductor, to outside of the fuel tank in a vapor state.