Liquid level detection device of liquid supplying facility, liquid level detection method of liquid supplying facility, and liquid supplying facility having the liquid level detection device

A liquid level detection device for a liquid supplying facility includes: a lower pipe which is in communication with a liquid-phase section of an interior space of a tank, the liquid-phase section being occupied by a liquid; an upper pipe which is in communication with a gas-phase section of the interior space of the tank, the gas-phase section being disposed above the liquid-phase section and occupied by a high-pressure gas; a differential pressure gauge for detecting a differential pressure between the lower pipe and the upper pipe; and a liquid level calculation part for calculating a liquid level of the tank on the basis of a differential pressure detection result of the differential pressure gauge. An inside of the upper pipe is filled with a filling liquid at least in a partial range.