During transportation, etc. of frozen or refrigerated foods, if these foods accidentally meet a temperature of the predetermined temperature or over, these frozen or refrigerated foods could inadvertently be deteriorated or decomposed. Therefore, strict temperature control management should be carried out to preserve the frozen or refrigerated foods. In the field of medical care, temperature control in preserving particular kind of medicinal drugs, blood, specimen, etc. play an important role. These products could deteriorate rapidly when the temperature rises, and such products may not be used if things go wrong.
For controlling such temperature, heat-sensitive indicators that irreversibly indicate a signal in a case where a temperature rises to or over a predetermined temperature, have been used so far. Checking these heat-sensitive indicators allows us to know whether or not the storage temperature during transportation exceeded the upper limit of a temperature control range. Such heat-sensitive indicators are disclosed in Patent Documents 1-3 described below. The heat-sensitive indicator disclosed in Patent Documents 1-3 comprises: a colored hot-meltable substance which melts at a predetermined temperature; an absorber which absorbs or is permeated with the melt of the colored hot-meltable substance; and a separating member such as separating membrane, etc. which maintains the colored hot-meltable substance away from the absorber without contacting to each other. At the time the heat-sensitive indicator is used, the separating member is broken under a temperature lower than the melting temperature of the colored hot-meltable substance, bringing the colored hot-meltable substance into contact with the absorber. When the colored hot-meltable substance is melted by raising the temperature of an environment in which the heat-sensitive indicator is placed, then the molten material of the colored hot-meltable substance is absorbed into the absorber, thereby the absorber is colored. The color on the absorber does not disappear even when the temperature of the environment is brought down to a temperature lower than the melting point of the colored hot-meltable substance.