The waterproof structure of an electrical junction box herein prevents water from entering into the electrical junction box. The electrical junction box includes a main body having a groove formed on an outer wall thereof and a tubular wire cover composed of front and rear split covers. Electric wires are passed downward through the wire cover and fixed to a lower portion of the wire cover using a tape. The wire cover is slidably engaged with the groove of the junction box main body. The waterproof structure includes a step portion as a water stop formed on a rear wall of the wire cover. The rear wall is arranged in contact with or adjacent to a back wall of the groove so that the step portion prevents water from entering into the electrical junction box from bottom along the back wall of the groove through the electric wire.