To provide a method and an apparatus for accurately measuring the level of molten metal and the thickness of a slag layer even in the case that slag is formed and that the thickness of the slag layer varies.

Microwaves 26 are made incident onto the surface 11 of the molten metal in which the slag 12 is floating on its surface. The intensity of the reflected waves of the microwaves is recorded with the passage of time. The time of a maximum peak corresponding to a peak A indicating a maximum value of the intensity of the reflected waves, and the small peak time of the first small peak B which occurs after the time of the maximum peak, are measured. On the basis of the distance that the microwaves travel during the time period between the time of incidence of the microwaves and the time of the small peak, the surface level of the molten metal is determined. On the basis of the distance that the microwaves travel during the time period between the time of the maximum peak and the time of the small peak, the thickness of the slag layer is determined.