PROBLEM TO BE SOLVED: To provide a lighting instrument structure of a traffic signal such that when a lighting instrument is formed of a steel plate, assembly operability is improved around insertion holes of side plates while high watertightness is maintained.SOLUTION: A lighting instrument structure of a traffic signal is formed of a steel plate and comprises a first lighting instrument 3 and a second lighting instrument 5 adjoining the first lighting instrument 3. A cylindrical flange 71 raised from a peripheral edge 44a of a first insertion hole 44 toward the second lighting instrument 5 is formed on a side plate 31 of the first lighting instrument 3, and a mortar-shaped flange 81 extending from a peripheral edge 144a of a second insertion hole 144 into the second lighting instrument 5 and gradually decreasing in internal diameter away from the first lighting instrument 3 is formed on a side plate 129 of the second lighting instrument 5. An annular seal member 83 is arranged on an outer peripheral side of
