To provide a vehicular fuel tank unit capable of suppressing storage of water on a top surface of a lid member.

The vehicular fuel tank unit comprises a lid member 21 for covering an opening part 11 of a fuel tank 10, a cylindrical member 12 extending from the circumference of the opening part 11 of the fuel tank 10 with a male screw part being formed on an outer circumferential side, and a nut member 30 with a female screw part formed on an inner circumferential side. The lid member 21 has a rib 261 against which a ring part 34 of a nut member 30 is pressed by fastening the female screw part to the male screw part. Thus, a part (a water draining part) having no rib 261 out of an upper surface of a flange 26 forms a space as a water draining passage 263 between a lower surface of the ring part 34 and itself. Water on the top surface of the lid member 21 flows into the female screw part 31 through the water draining passage 263, and is discharged through a backlash space.

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