To retain the heat of foods stored in a lunch box for a long period of time in a simple and inexpensive constitution.

This lunch box with the heat-retaining function is formed with a lunch container 10 having a boiled rice inner chamber 11 for storing at least the boiled rice, a lid body 14 for covering the lunch container 10, and a heat-storage body storing chamber 15 formed in at least a lower stage of the boiled rice inner chamber 11, and detachably laid with a heat storage body 21 for at least retaining the heat of the boiled rice stored in the boiled rice inner chamber 11; and the heat storage body 21 stored in the heat storage body storing chamber 15 previously stores heat by heating from the outside and radiates the stored heat with time.