To galvanize (electro-galvanizing, hot dip galvanizing) the surface of steel plate, it is required to meet the conditions of the surface cleanness for good looking and adhesion. Alkaline degreasing agent (permeability, prevention of secondary pollution, emulsifiability, low-temperature degreasing, etc.) is applied to the steel surface through pressure-spray, brush roll based physical degreasing, or oxygen & hydrogen based ECT process using electrochemical properties, in order to purify the surface. For the process, it is possible to consist of the centrifugal separator to separate a degreasing agent from emulsified oil, the automatic washer of spray nozzle, oscillation of brush roll, the equipment for long life span of ECT electrodes, and ozone based degreasing device.