To visually confirm temp. history at a time of low temp. control and to facilitate temp. control by mixing a chelatable reagent with a soln. containing metal at the normal temp. to form a metal chelate complex to exhibit a color state.

A transparent plastic container 1 is partitioned by a sector plate 2 and a chelatable regagent A is housed in a lower chamber and a solution B containing metal forming a chelate complex by the reagent A is housed in an upper chamber. Since the sector plate 2 partitions the upper and lower chambers tightly, the reagent A and the soln. B are not mixed with each other. The container 1 is hermetically closed by a reversal type upper lid 5 and a projection 6 protruding in close vicinity to the sector plate 2 is provided to the upper lid 5. When the upper lid 5 is pushed by a finger, the projection 6 pushes the sector plate 2 and the plate 2 attached in a fragile state falls. Therefore, the reagent A and the soln. B are mutually mixed to form a color by a metal chelate complex soln. A+B. When food preserved at low temp. receives history of temp. higher than the low temp., the soln. A+B is discolored to know the temp. history of preserved food.