The practical frying test was carried out in a automatic test fryer of indirect heating system using Kanechlor (chlorinated biphenyl) arranged on the basis of the model examination by the continuous water-spraying and heating system, and the effect of protection of fat surface was examined using soybean oil and pieces of raw potato as frying materials (Fig.-1, 2, Table-1). In order to get good reproducibility, the conditions introducing the frying materials, the temperature of fat entering into the cooker, the rate of fat circulation and frying time were strictly controlled. As shown in Fig.35, Table-2, it was observed that the thermal oxidative deterioration was prevented effectively by means of metal float or hood, but the hydrolytic deterioration was not prevented. Colour development and lowering of fat stability to autoxidation resulted from hydrolytic deterioration. These tendencies were essentially the same as that of the model examination. However, the increase of acid value was very slow in comparison with the model examination. When soybean oil of high acid value (about 2.0) was examined, the acid value increased considerably in a rapid rate in either cases with protection or without protection (Fig.-6). From this result, in order to get good frying quality, some measure to prevent the hydrolysis of fat should be taken in addition to the protection of fat surface. As a measure of protecting fat hydrolysis, the effect of cyclopropenoid fatty acid was examined (Fig.-7). By the addition to soybean oil, of 10% of refined kapok seed oil which contains 11.4% of cyclopropenoid fatty acid, a good stability against hydrolysis was observed with light colour development and good stability to autoxidation. Accordingly, when the fat surface in the fryer is protected by means of metal float or hood, and in addition the means to prevent fat hydrolysis are provided, the both of thermal oxidative and hydrolytic deteriorations are practically prevented, and good frying quality will be thus maintained during deep fat frying.