In a lid member with a waterproofing function, the lid member including a projection portion integrally formed in one surface of a resin main body of the lid member opening and closing an opening portion formed in a housing, and accommodated in the opening portion, and a seal portion integrally formed in an outer peripheral surface of the projection portion, provided with an annular projection coming into close contact with a peripheral surface of the opening portion, and made of a rubber-like elastic material, the annular projection is formed as a shape which is expanded toward an inner portion side of the opening portion, forms in its outer peripheral surface a taper surface which converges toward the inner portion side, and is provided with a seal surface having a circular arc shaped cross section so as to be connected to a maximum diameter portion of the taper surface.