A method and an arrangement for monitoring a pressure sensor are suggested which determines a value representing the ambient pressure of an internal combustion engine. Each of the groups of light beams deflected by the optical deflecting part passes through at least two scanning lenses while being directed to a corresponding one of the scan surfaces. or less under atmospheric pressure. The present invention is able to eliminate the influence by the slag layer floating on the molten metal and to achieve stably and continuously accurate measurement of molten metal level by using the measuring probe inserting into molten metal through the slag-metal interface and having the slag thickness information after lifted.