Experimental studies for reducing the refining period, especially decarburization period, has been carried out with a test converter of 5ton, applying top blowing of the mixed gas of nitrogen and oxygen throughout decarburization period to aimed carbon concentration of less than 0. 05% in addition to bottom blowing through tuyeres. The method of mixed gas top- and bottom-blowing has been confirmed to be effective for reducing decarburization period without any additional oxidation loss of chromium, and hence at the same chromium yield as that of the conventional process. Mass transfer of carbon in steel melt is a rate determining step for the decarburization reaction in the low carbon concentration range. In order to clarify the mechanism of enhancement of decarburization rate by top- and bottom-blowing with mixed gas, cold model studies on mass transfer have been done with the use of air and water.