In accordance with the present invention, a ventilating structure for a steel box comprises: a metal box in which an upper plate, a lower plate, and side plates are welded in a box shape; a plurality of square pipes arranged and fixed on the upper surface of the metal box at intervals; and a plurality of decks intersected and continuously fixed on the upper surfaces of the square pipes to enable pedestrians to walk or pass the decks. The ventilating structure further comprises: a ventilation pipe which has a length, is installed in the metal box while penetrating the upper plate of the metal box by a length, and allows air to flow through an upper opening and a lower opening; a roof plate which is located at the top of the ventilation pipe while covering the upper opening of the ventilation pipe located between the deck and the upper plate of the metal box, and prevents rainwater from entering the upper opening of the ventilation pipe from the gap between the decks while allowing the movement of air; a connecting rod extending from the lower surface of the roof plate to the outside of the lower opening of the ventilation pipe located between the upper and lower plates of the metal box; and a wing plate which is formed on a lower end of the connecting rod and allows air moving through the ventilation pipe to diffuse into the space between the upper and lower plates of the metal box. Therefore, condensation and corrosion can be prevented.