In a filler pipe assembly, a holder member secured to an inner wall of a filler pipe is provided with an outer profile of a circular cross section substantially conforming to an inner peripheral wall of the filler pipe and an inner profile defining a bore of a circular cross section eccentric to the outer profile thereof. A relief valve is provided in a part of the holder member defined between the outer profile thereof and the bore owing to an eccentricity of the bore relative to the outer profile. Owing to the eccentric relationship between the outer profile and inner bore of the holder member, a space for a relief valve can be provided in the area where the distance between the inner wall of the bore and the outer profile is the greatest, and this allows the relief valve to be mounted without complicating the structure.