In the earthquake disaster management plans of most of the municipalities in Japan, the hazard to evacuation of city residents due to post-earthquake fires is estimated usually with regard to effect of radiation, but due attention is seldom paid for fire flow induced by city conflagration. Concerning the evacuation in urban fire, we should seriously pay attention to the hazard due to the fire flow that is blown over by a high wind and covers over a wide urban district. The tolerable exposure limits of human body to its elevated temperature and poisonous gases are very low. In addition the hazards caused by the hot smoke are considered to render considerable difficulty in fire fighting as well, so may affect the fire spread itself. The final goals of this study are to analyze the causes of heavy toll of human lives that claimed by many urban fires in Japan in the past and to find effective measures to mitigate the hazard due to urban fire. For the first step, this paper aims at establishing a methodology to assess the hazard area due to fire flow in an urban conflagration in the past. More specifically, on the basis of an urban fire spread chart of Sakata City Fire, which broke out in 1976, we reconstruct the area of fire spread and estimate the temperature rise in the area due to the fire flow with the elapsed time after the onset of the fire.