PURPOSE: To improve the efficiency of the installing operation of a loading and air treating unit by installing the unit such as an ethylene removing unit, etc., without restricting in the loading operation and the installation place.

CONSTITUTION: A mounting plate 24 mounted at the lower end of a mounting trestle 20 in which an air treating unit 120 is installed is so installed as to cross over a pair of adjacent upper plate 15a of T-shaped sectional ventilation rails 15 aligned at the bottom in a box Q. A fixed plate 25 is brought into contact with the lower surfaces of the upper plates 15a of the pair of the rails 15, the pair of the plates 15a of the rails 15 are sandwiched between the plate 25 and the plate 24, and clamped by bolts 26.