PURPOSE: To detect distribution of the layer thickness and the layer thickness ratio of charged material with high accuracy, by detecting a surface sectional shape of a charged material charged successively in a vertical furnace batchwise and performing a prescribed operational processing.

CONSTITUTION: A laser beam 7 of a laser apparatus 6 is irradiated to the inside of a blast furnace 1 with a reflex mirror 9 and a reflected light 7a from the surface of a charged material 2 is detected by a photodetector 8. A surface sectional shape of the material 2 is measured by a light projection angle α of the beam 7, a photodetecting angle β of the light 7a and a distance L between a light projection window 4 and a photodetecting window 5. A sedimentation velocity distribution dn of n-th time's charged material is found by the difference of a curve A-A of the surface sectional shape immediately after charging n-th time's charged material and a curve B-B of the remeasured surface sectional shape immediately before charging (n+1)-th time's charge and each difference of the measuring time. Next, a curve C-C of the surface sectional shape immediately after (n+1)-th time's charge is measured and B'=B+(tB- tC)dn is computed from its measuring time tC and a measuring time tB of the curve B-B. The layer thickness distribution of (n+1)-th charged material is found from the difference of both curves. The layer thickness ratio distribution is found by repeating said process.