Flow field due to thermal convection upon a ring type heat source is studied numerically and experimentally, which simulates hopefully the flow due to the devastating combustion just after the Great Kanto Earthquake. New numerical scheme was developed, which enables us to treat accurately singular or nonuniform grid system in finite-difference method. In experimental measurements, extremely low velocity and its fluctuation were detected by laser doppler velocimeter. Both results show the main flow due to the convection is strongly concentrated within the central portion above the ring where no heat is supplied at the bottom. Conspicuous unsteadiness was observed, which is considered to be the main cause of the tornado at the calamity of the Great Kanto Earthquake.