A seal structure includes: a tubular protrusion communicating with an inside of a resin tank body and projecting from the tank body; a mouth member being fixed along an inner peripheral surface of the protrusion; a lid member including a second tubular portion inserted from one end into the mouth member, a flange portion covering the other end of the second tubular portion, and a third tubular portion extending toward the insertion direction of the second tubular portion; a supporting surface provided to an outer peripheral side of the protrusion or the fuel tank body; and a ring-shaped sealing member supported on the ring-shaped shoulder surface and held between an outer peripheral surface of the protrusion and an inner peripheral surface of the third tubular portion in a state where the lid member is attached to the mouth member.