PURPOSE: To simplify maintenance and to prevent gas leak in such a way that in a discharging device for liquefied gas tank in a tank lorry or the like, an enclosed chamber having a built-in drain pump is installed in the center of the tank bottom part, and the liquefied gas in the chamber is substituted by inert gas.

CONSTITUTION: A lid plate 20 is attached in liquid-tightness and in free command of attaching to and detaching from a bottom part central opening part 12 of a tank 10, a dip pump 26 is solidly installed on the inside of the lid plate 20, a housing 16 which can closely surround the pump 26 is tightly installed in the tank 10, and a valve device 18 which freely conducts its inside and outside is positioned operably on the outside on one side of the housing 16. In case of dismounting the pump 26, liquefied gas is turned out through a drain port 48 by filling inert gas through an inert gas supplying port 46 installed on the lid plate 20 into the inside of the housing 16, and pump maintenance operations can be easily and safely performed by cutting off and separating the piping system of the pump 26, ad then by dismounting the lid plate 20.