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Introduction.The KEK-RIKEN group has recently conducted a series of experiments probing the inner structure of a volcano by measuring the intensity attenuation, track by track, of near-horizontal cosmic-ray muons that pass through the volcano. 1)-3) Several innovative technical developments were carried out in order to realize successful measurements of the volcanic innerstructure and its time-dependent behavior by using near-horizontal cosmic-ray muons: a) one detection system for a track determination comprises 2 sets (1.5 m apart) of position sensitive detectors, each of which is 10 segmented (each, 10 cm × 1 m × 3 cm thick) plastic scintillation counter pairs of x-y coordinate determination and, as seen in Fig. 1a, two systems with 2 m × 1 m sensitive area in total were used; b) accumulation of F (towards object) / B (towards sky) ratio data at each track of muons for self-normalization of the weak nearhorizontal cosmic-ray events; c) multiplicity-event-cut through iron-plate for soft-component background removal. The sound operation of the system for a longterm (several months) measurement was confirmed in the measurements both at Mt. Asama 2) and at Mt. WestIwate.3)Experimental arrangement. As indicated earlier, 4) the radiography by using near-horizontal cosmic-ray muon should be useful for a large-scale industrial machinery like a blast furnace. By using the same muon radiography set-up as used for the volcano research, under a Collaborative Research Contract between KEK and Nippon Steel Corporation, the experiment has been carried out to probe the inner structure of the world's largest blast furnace (Blast Furnace No. 2, DaiNi-Kouro) at the Oita Factory of the Nippon Steel Corporation during a time of its full operation. As seen in Fig. 1b, the detection system was placed at the horizontal position of 17.3 m from the center of the furnace (nearly equal to the diameter of the furnace) and at the vertical position of, at first, at 2.7 m from the ground (the brick base is occupying from 4.97 m to additional a few Abstract: By using the detection system of the near-horizontal cosmic-ray radiography originally developed for probing inner structure of volcanic mountains, a measurement was conducted to probe the inner structure and its time-dependent change of the blast furnace for iron-making. Precise determination (± 5 cm) of the thickness of brick used for both base-plate and side-wall was made in 45 days; a crucial information to predict a life-time of the furnace. Also, the local density of iron-rich part was determined in ± 0.2 g/cm 2 in 45 days; static structure as well as time-dependent behavior can be monitored for the iron-rich part of the furnace during operation.

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