To provide a Kotatsu heat retaining frame which allows a user to put into/out his or her feet therefrom easily, gives little sense of incompatibility when the user touches the feet of the frame, has heat retaining properties and can be used in almost all the low table-type Kotatsu, when an existing low table-type Kotatsu is used as a seating fixed Kotatsu.

In the heat retaining frame, a rubber string (elastic body) 1 is spread over the topmost part of four heat retaining frame supports 3 to secure easiness with which a user puts his/her feet into/out of the frame and heat retaining properties, and a mattress is covered thereon. In order to be available in almost all the existing low table-type Kotatsu, each of the four sides of the Kotatsu includes a size adjusting frame piece 4 to vary the size of the heat retaining frame. Furthermore a caster 6 is attached to each of the four corners at the bottom of the heat retaining frame to allow the user to easily put his/her feet into/out of the frame.