PURPOSE: To perform accurate level measurement even though noises are included in a photograph signal, by converting a scanning line position signal, which has the highest frequency component that is judged among all of the scanning lines of a two dimensional solid state photographing sensor, into a preset distance signal.

CONSTITUTION: Laser light is projected on the surface of fused steel 1 at a required angle θ1. The reflected light is guided to a two dimensional solid state photographing sensor 3b, whose light receiving surface is slanted by a required angle θ with respect to the light axis of a condenser lens 3a, and a photograph signal is obtained. The frequency of the photograph signal is analyzed by a frequency analyzer 5 for every scanning line. The position of the scanning line having the highest frequency component is judged by a pattern judging device 6. The result is converted into a distance signal (a) to the surface of the fused steel 1, which is preset by a function generator 7. In this way, even though noises due to dust, scale, and the like are included in the photograph signal, no effect is given to the power of the frequency component on the scanning line. Therefore the accurate level can be measured.