To easily detect the moving of a side plate of an inner tank in the inspection of a released point by inserting a probe into an inner cylinder allowed to follow the moving of a diaphragm from outside of a low temperature storage tank to get the tip thereof set in contact with the side plate of the inner tank.

In a low temperature storage tank 10, an inner tank 12 is provided inside an outer tank 11 through a cold insulating space 13, which 13 is sectioned by a pearlite diaphragm 14. An outer tank side space is filled with a pearlite 15 as cold insulating member and an inner tank side space with a glass wool 16. In a jig 20, an inner cylinder 22 is fitted into an outer cylinder 21 to be build in a double structure. When inspecting a released point of the low temperature storage tank 10, the glass wool 16 filled inside is removed to insert a bar-shaped probe 30 with a specified length from an external end part of the outer cylinder 21 and the tip thereof is set in contact with the side plate 12A of the inner tank. When the temperature in the low temperature storage tank 10 rises and the inner tank 12 is thermally expanded to make the side plate 12A of the inner tank move outward, the probe 30 also moves eventually. The degree of the displacement of the side plate 12A of the inner tank can be learned by measuring the movement of the probe 30.