Gas sensing element, $alpha-Fe_2O_3$ was synthesized by dehydration, reduction, and oxidation of $alpha-FeOOH$, which was synthesized with $FeSO_4cdot 7H_2O$ and NaOH. They were produced as a bulk-type, a thick film-type. Then, their responses and mechanisms of response to the gas of carbon monoxide were studied. The qualities of gas sefising elements are decided by the structure and the relative surface area. In the process of $alpha-FeOOH$ synthesis, the effects of reaction conditions as the equivalent ratio, on the structure and the relative surface area of gas sensing element were observed. The changes of the structure were measured with XRD, SEM,TG-DTA and BET. The resistance changes of the synthesized gas sensor in the air were measured. The response ratio were also measured for the changes of working temperature and gas concentration. As a result of analysis with XRD, it was confirmed that the the best conditions for the synthesis of $alpha-FeOOH$ were equivalent ratio 0.65. The thick film-type element of $gamma-Fe_2O_3$ responded more quickly than the bulk-type did. The structure and the relative surface area of the $ ho-FeOOH$ were confirmed as the important factors deciding gas response charcteristics.