To provide a heat reservoir which can keep a predetermined temperature for a long time and by which heat reservation work can be carried out in a short time without loading labor on a worker.

Because a latent heat reservoir 12 is configured to reserve heat by utilizing latent heat generated by phase change, a predetermined temperature can be kept for a long time. A heating member 11 can heat an object by an electromagnetic induction heater IH and comprises a metallic material excellent in heat conductivity including a part of a metallic fiber group 11b and further the latent heat reservation material 12 is set so as to get into gaps between metallic fibers of the metallic fiber group 11b of the heating member 11, the heating member 11 including the metallic fiber part group 11b is entirely heated in a short time by the electromagnetic induction heater and heat is efficiently applied on the latent heat reservoir 12 from the heating member 11 corresponding to the contact area. Accordingly the heat reservation work can be performed in a short time without loading labor on the worker.