A defect detection apparatus for a steel plate comprising: a magnetized portion generating magnetic flux through first and second magnetization poles (211, 212) to magnetize a steel plate being driven during use of the defect detection apparatus in a driving direction relative to the magnetized portion, the steel plate having an upper surface above which the magnetized portion is disposed and a lower surface opposite the upper surface, and the magnetized portion magnetizing the steel plate in the said driving direction; and a leakage flux detection unit detecting a density of leakage flux leaking when the generated magnetic flux passes through a defect; wherein the leakage flux detection unit includes: a first leakage flux detection unit disposed to be spaced apart from a central position below and between the first and second magnetization poles (211, 212) in the driving direction of the steel plate or in a direction opposite to the driving direction, and disposed adjacent to the lower surface of the steel plate; and a second leakage flux detection unit disposed below at least one of the first magnetization pole or the second magnetization pole, and disposed adjacent to the upper surface of the steel plate, the defect detection apparatus being characterized in that the first leakage flux detection unit is disposed in a position to maximize an absolute value of a differential value of a magnetic flux component in the magnetic flux generated by the magnetized portion, in a direction perpendicular to the steel plate with respect to the driving direction of the steel plate. a magnetized portion generating magnetic flux through first and second magnetization poles (211, 212) to magnetize a steel plate being driven during use of the defect detection apparatus in a driving direction relative to the magnetized portion, the steel plate having an upper surface above which the magnetized portion is disposed and a lower surface opposite the upper surface, and the magnetized portion magnetizing the steel plate in the said driving direction; and a leakage flux detection unit detecting a density of leakage flux leaking when the generated magnetic flux passes through a defect; wherein the leakage flux detection unit includes: a first leakage flux detection unit disposed to be spaced apart from a central position below and between the first and second magnetization poles (211, 212) in the driving direction of the steel plate or in a direction opposite to the driving direction, and disposed adjacent to the lower surface of the steel plate; and a second leakage flux detection unit disposed below at least one of the first magnetization pole or the second magnetization pole, and disposed adjacent to the upper surface of the steel plate, the defect detection apparatus being characterized in that the first leakage flux detection unit is disposed in a position to maximize an absolute value of a differential value of a magnetic flux component in the magnetic flux generated by the magnetized portion, in a direction perpendicular to the steel plate with respect to the driving direction of the steel plate. a first leakage flux detection unit disposed to be spaced apart from a central position below and between the first and second magnetization poles (211, 212) in the driving direction of the steel plate or in a direction opposite to the driving direction, and disposed adjacent to the lower surface of the steel plate; and a second leakage flux detection unit disposed below at least one of the first magnetization pole or the second magnetization pole, and disposed adjacent to the upper surface of the steel plate, the defect detection apparatus being characterized in that the first leakage flux detection unit is disposed in a position to maximize an absolute value of a differential value of a magnetic flux component in the magnetic flux generated by the magnetized portion, in a direction perpendicular to the steel plate with respect to the driving direction of the steel plate.