The state of slag foaming generated in an electric furnace steel manufacture is judged by measuring the NOx amount in exhaust gas. The foaming state is adjusted to completely interrupt the contact of melted steel with air so that low nitrogen of the steel is always achieved or reduction of electric power consumption rate is achieved. Thus, in the manufacture of steel by subjecting iron scrap to dissolving, refining and heating stages successively in an electric arc furnace, the NOx amount in the exhaust gas is measured at the refining and heating stages of the melted steel and then the state of the slag foaming is judged depending upon the measured data.