Provided is a pressure vessel for high-pressure hydrogen gas capable of large-capacity hydrogen storage with a single pressure vessel while being easy to produce, transport, and install. A pressure vessel for high-pressure hydrogen gas comprises a steel container that is composed of two or more steel pipes or tubes connected by screw threads.

There is provided a method for storing a fluorine-containing nitrogen compound that prevents the progress of a reaction of the fluorine-containing nitrogen compound during storage. A fluorine-containing nitrogen compound containing nitrosyl fluoride and nitroyl fluoride contains or does not contain at least one type among sodium, potassium, magnesium, and calcium as metal impurities. When any of the foregoing is contained, the fluorine-containing nitrogen compound is stored in a container with the total concentration of the sodium, the potassium, the magnesium, and the calcium set to 1000 ppb by mass or less.

There is provided a method for storing fluoroalkene in which, even when stabilizers for suppressing reactions of fluoroalkene are not added, the reactions of the fluoroalkene hardly occur and the purity of the fluoroalkene hardly decreases during storage. Fluoroalkene having a bromine atom or an iodine atom in the molecule is at least one type of fluoroethylene represented by a first general formula C.sub.2H.sub.pF.sub.qX.sub.r and fluoropropene represented by a second general formula C.sub.3H.sub.3F.sub.tX.sub.u (X represents a bromine atom or an iodine atom), and contains or does not contain at least one type among manganese, cobalt, nickel, and silicon as metal impurities. When any of the foregoing is contained, the fluoroalkene is stored in a container with the total concentration of the manganese, the cobalt, the nickel, and the silicon set to 1000 ppb by mass or less.