To improve use efficiency, by making maintenance or replacement of an ultrasonic sensor capable with a charge condition of LPG or use of gas remain continued.

This liquid level measuring device, constituted by providing an ultrasonic sensor 4 in the head part of a vessel 2 for measuring its liquid level 3a of LPG3, is provided with a pipe unit 7, formed by connecting the ultrasonic sensor 4 capable of mounting/demounting to the upper end to make the lower end communicate with in the vessel 2 and providing an opening part 6 communicating with a gas take-out valve 5 in a side part, and a ball valve 8 provided in almost the central part of this pipe unit 7 to perform switching by turning among a fully-opened condition connecting to each other the ultrasonic sensor 4, a vessel inner part 2a, and the opening part 6, a fully-closed condition closing a side of at least the vessel inner part 2a, and a partially- opened condition closing a side of the ultrasonic sensor 4 and connecting to each other the vessel inner part 2a and the opening part 6.