A waterproof structure for an electrical wiring instrument includes electrical wires, a housing, an upper cover and rubbery elastic members resulting from a pair of rubbery elastic materials. The housing has a cover portion formed on an end of the housing and provided with first accommodation grooves for accommodating the electrical wires. Each of the first accommodation grooves is defined by a pair of first ribs parallel to each other. The upper cover has second accommodation grooves formed for accommodating the electrical wires. Each of the second accommodation grooves is defined by a pair of second ribs parallel to each other. In assembling, after accommodating the wires in the first accommodation grooves, the wires are interposed between the pair of rubbery elastic materials in the form of sheets. Then, the upper cover is laid on the cover portion and subsequently subjected to ultrasonic oscillation under pressure. Consequently, the pair of rubbery elastic materials are cut by the first and second ribs to produce the elastic members surrounding the wires in accommodation holes defined by the first and second accommodation grooves.