It is well known that cosmic-ray muons are illuminating every substance on the earth. Muons arriving vertically from the sky have an intensity of one muon every square centimeter every minute with a mean energy of a few GeV. It is also well known that muons with a smaller average intensity, but with a higher intensity at energies higher than a few hundred GeV, are arriving horizontally along the earth. For the purpose of probing the internal structure of a volcano, these horizontally arriving muons can more easily be used, provided that the muon flux is reasonably high and that the size of the detection system is realistic. An improved cosmic-ray muon detection system comprising two detectors which are vertically segmented into 10 channels and horizontally segmented into 10 channels, with multiplicity cut of the soft- component background of cosmic ray was developed. The multiplicity cut method eliminates soft components by using cascade shower generation through an intermediate iron absorber. By applying to the measurement in internal structure of the volcano Mt. Asama, we clearly detected the hidden crater in the vicinity of the top. There, we proved that the volume occupancy in the region of a crater is less than 30 % by comparing to the detailed Monte Carlo simulation. The future prospect of the present method is also discussed.