To provide a manufacturing method of a steel plate and a surface layer hardness measuring device for magnetic material capable of suppressing a hard spot while suppressing the lowering of productivity.SOLUTION: A manufacturing method of a steel plate includes: a rolling process of rolling a steel piece to obtain the steel plate; a surface layer hardness measuring process of measuring a surface layer hardness and judges a portion harder than a surface layer hardness preset with respect to a surface layer of the steel plate as a hardness indication portion based on the measured surface layer hardness of the steel plate; and a removal process of removing the hardness indication portion which is judged on the surface layer of the steel layer. Therein, the surface layer hardness measuring process has a demagnetization process of performing demagnetization of the surface layer of the steel plate and an electromagnetic measuring process of measuring electromagnetic characteristics of the surface layer of the steel plate after the demagnetization process and measuring the surface layer hardness of the steel plate from the measured electromagnetic characteristics.SELECTED DRAWING: Figure 1
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