To provide a seal structure of an oil filler port box which can prevent entry of water even when being subject to high water pressure and achieves improvement of sealability between an oil filler port box and a vehicle body.SOLUTION: In a seal structure of an oil filler port box 10, the oil filler port box 10 has a box body part 13 and a box flange part 11. A box seal member 20 is attached to a periphery of the box flange part 11. The box seal member 20 has: a seal member body part 21 attached to the box flange part 11; a first seal part 22 which is formed at the seal member body part 21 and contacts with a vehicle body flange part 3; and a second seal part 26 which is formed at a tip of the seal member body part 21 and contacts with the vehicle body flange part 3 or the vehicle body 2 which is located at the vehicle outer side of a portion with which the first seal part 22 contacts. A portion of a tip of the first seal part 21 which contacts with the vehicle body flange part 3 is formed so as to have a cross section formed into an arc shape.SELECTED DRAWING: Figure 4
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