Due to comprehensive research on floor heating in Japan and abroad over the last twenty years it is possible today to predict these combinations of comfortable conditions. The specific aim of this investigation is to obtain experimental data upon which the comfortable range of the floor surface temperature could be based, thereby providing a practical and more satisfactory design guide. Accordingly literatures on floor heating are collected and investigated with the purpose of revealing what the comfort range of floor surface temperature ought to be. Authors tried to obtain a comfortable and permissible floor surface temperature and then reconfirm the maximum comfortable range so that they can propose a reasonable comfortable floor surface temperature and make it fit for practical and extensive use. After synthesizing all the comfortable and permissible values reported in many other papers, this paper finally advances a suggestion on the optimal and permissible floor surface temperatures. Results concerning this subject are as follows. The lowest floor surface temperature would be limited to 25 degrees C, and it is 21-23 degrees C for the air temperature so that more than 80% subjects can be expected to be comfortable within this recommended temperature interval.