In order to measure the thickness of the slag layer on the molten metal in the melting vessels for steel making foundaries, we manufactured the slag-thickness measuring system consisting of the probe and its driving system, which is based on the principle of detecting the change in the electrical resistivity. Experimental results show that there is a very wide difference of the electrical resistivity by the order of about $10^3$ between the molten metal and the hot slag and, hence, the air/slag and slag/molten metal interface can be exactly detected with the use of the probe which can be moved toward the melt. Therefore, it can be concluded that the proposed scheme of the slag measurement system in this study is Proven to be excellent in its efficiency and accuracy.