To provide a heat radiating duct constituted to suppress dust from intruding into the interior.

Heating components such as fluorescent lamps are housed in the interior of a flat plate part 11 of a metal plate or resin plate, cut erected pieces 12 are formed on the flat plate part 11, and the top ends 12a of the cut pieces 12 project forwards over the peripheral edges 13a of the openings 13 to discharge heat from the openings 13. The top ends 12a of the cut pieces 12 are disposed and superposed on regions contacting the openings 13 and hence dust hardly intrudes through the openings 13.