Since the defect density of slowly pulled Czochralski (CZ) Si crystals is fairly low, determination of the surveillance volume is very important to get correct information about defects in the crystals. Here, the volume is defined as that to be searched by a single surveillance trial from a viewpoint of economical quality control of the crystals and should be equal to or a little larger than the reciprocal density of the defects. The following two-step investigation resulted in successful and novel use of an ordinary IR light scattering tomograph (LST) instrument for this purpose. (1) The surveillance thickness of photoluminescence (PL) due to carriers optically injected by a laser beam for LST is estimated as 300 μm in the crystal, because the carriers diffuse out emitting PL about 150 μm from the position illuminated by the laser beam. Thus PL mapping is very effective to search for low defect density silicon crystals since defects such as dislocations act as PL killer centers. (2) The dark positions in the PL mapping were carefully surveyed by layer-by-layer tomography using the light scattered by defects, where a beam of 1.06 μm radiation from a Nd-YAG laser was focused into about 5 μm in diameter to scan inside the crystal. By complimentary combination of the above two methods, large entangled dislocation loops were clearly confirmed in a slowly pulled p-type CZ Si crystal.