An apparatus for detecting inner defects of a steel plate comprises an all-defect detection unit for detecting all defects including surface defects present on the surface of the steel plate and inner defects present inside the steel plate by intensity of leakage flux, measured by generating magnetic flux in a strike direction of the steel plate; a surface defect detection unit for detecting surface defects by intensity of leakage flux, measured by generating magnetic flux in the thickness direction of the steel plate, for a predetermined detection region including all the defects detected by the all-defect detection unit; and a data processing unit for detecting only inner defects present in a detection region by excluding the detected surface defects from all the defects detected by the all-defect detection unit, for the detection region, thereby detecting only the inner defects of the steel plate.