To provide a support frame of a lightweight Kotatsu (Japanese foot warmer) frame, and to provide a side part forming the support frame.

This support frame for the Kotatsu frame is composed of four side parts H and connection parts connecting the respective side parts H. The elongated side part H is composed of a synthetic resin elongated base body 16 and a synthetic resin elongated lid body 17. The base body 16 has a box shape including an opening in the upper face. When fitting the lid body 17 such that the opening is closed, the base body 16 and the lid body 17 are assembled to form the side part H. In side the side part H, frame structures are formed of a projecting stripe 24 and reinforcement ribs 21 in a plurality of positions, and vertically-extending support pillars P are erectly provided by the fitting of fitting projecting parts 22 and fitting tube parts 25. The frame structures and the support pillars P enhance strength of the side part H.