PURPOSE: To detect the inside shape of a softened and welded zone in a blast furnace by discriminating the melting temp. of a metallic strand by the de-energization arising from the fusing of the exposed closed end of the metallic strand as the temp. of the fusing position thereof in the furnace and plotting the fusing positions in the furnace to obtain a temp. distribution curve.

CONSTITUTION: A measuring probe 1 is formed by embedding the metallic strand 3 having a known melting temp. in a packing material contained in a metallic protective sheath 2 in such a manner that the metallic strand turns back in the packing material. Such metallic strand is housed in the probe 1 and the closed circuit end 3a of the metallic strand 3 is exposed from a detecting hole 6 in the proscribed position of the probe. The probe is used in the state of energizing the metallic strand from the open circuit end 3b thereof. Such measuring probe is imposed radially on the charge in the furnace and is lowered together with descending of the charge. The melting temp. of the metallic strand 3 by the de-energization arising from the fusing at the exposed closed end of the metallic strand is discriminated as the temp. in said position in the furnace. The temp. distribution curve is obtd. by plotting the respective positions in the furnace, by which the condition in the furnace is detected.