A solidification process of 6-t slab ingots poured at high or low temperature is investigated by means of radio-isotope, temperature measurement, and direct sampling of mushy zone. The accumulation mechanism of large non-metallic inclusions in the bottom equiaxed zone of a ingot is described. The accumulation of large non-metallic inclusions in the bottom part of ingot relates closely with the phenomena occurring in the retained molten steel in the ingot, especially with a rapid development of mushy zone. In the case of ingot poured at low temperature, the growth of mushy zone is rapid owing to the crystallization of crystallites and their falling down to the bottom of ingot, which are remarkable at an early stage of solidification. This rapid growth of mushy zone prevents the floating of suspended inclusions and traps them to accumulate at the bottom part of ingot. The floating of inclusions trapped at the mushy zone of ingot would be difficult because of high apparent viscosity of the mushy zone. And they would grow to macroscopic size during the mushy state and the growth proceeds until completion of solidification. In the case of ingots poured at high temperature, on the other hand, a few crystallites crystallize and fall down at an early stage so that the mushy zone grows slowly. Inclusions would not be trapped but float out because of low viscosity of molten steel. This makes sound ingots. Since the formation of macroscopic inclusions depends on the rate of mushy zone development, macroscopic inclusions are few at columnar zone that is formed without precedence of the mushy stage even at the bottom of ingot, and they are rich at bottom equiaxed zone that is formed with being preceded by the mushy stage.