To provide a cold insulation tool capable of determining at a glance whether a cold storage medium is frozen sufficiently not only in a surface but also up to an inside thereof or not.

In this cold insulation tool A filled with the cold storage medium 2 using melting latent heat in a sealed hollow container 1, a cavity column 3 is provided to penetrate vertically through the substantial central part of the container 1, and a transparent material 4 having a freezing temperature lower than that of the cold storage medium 2 and getting cloudy when frozen is filled in the cavity column 3 to be sealed covered by a transparent lid 5. A visible piece 6 is preferably stored (engaged forcibly in the vicinity of the central part) therein in the filled transparent material 4. A high concentration of aqueous magnesium nitrate solution is preferable as the used transparent material 4.