To determine whether or not a cast slab can be transported to a hot rolling process in an HCR process according to the existence or absence of internal crack defects or their amount by quickly detecting the internal crack defects of the cast slab manufactured in a continuous casting process.

A specimen 6 is prepared by cutting off a part of a cast slab 2 just after casting in the continuous casting process 1 for every manufacturing lot in which the type of steel and casting conditions are identical. A cross sectional macroscopic test which can quickly achieve a result is performed for this specimen 6. Whether or not the cast slab 2 can be transported to the hot rolling process is determined according to the existence or absence of the internal crack defects or the amount of the internal crack defects measured by the cross sectional macroscopic test.