A junction box includes a box body and a conductive bar. A guide plate extends from a lead insertion opening, with a laterally extended connection portion at its extended end to connect the box body. First and second support plates and edges of the guiding plate form passages accessible to a lead insert space defined by the guiding plate, the connecting portion, the second support plate and the box body. The conductive bar has a bent conductive arm that moves into the inside of the lead insert space by an elastic force, therefore locked in said space, after it passes the passages.