A method of shot-peening (hereafter, "SP") a work to be treated, which work is a steel product that was treated by heat hardening, comprising: measuring the product with a non-destructive inspection using an eddy current sensor prior to a first SP treatment; the first SP treatment that removes a compound layer that was produced by the heat hardening; and a second SP treatment that gives compressive residual stress to a first-SP-treated surface that was treated by the first SP treatment; wherein the second SP treatment is carried out only on the products that have been proved to be acceptable by a non-destructive inspection and show that the compound layers have been removed from the first-SP-treated surface, wherein whether the products have been proved to be acceptable by the non-destructive inspection and show that the compound layers have been removed from the first-SP-treated surface is determined by the nondestructive inspection using the eddy current sensor, wherein the product is determined to be acceptable by the following: - first to measure both the phase difference (time) and the amplitude difference (voltage) of the current generated from the flux of alternating current at a plurality of points of the non-treated and treated work to be treated and - to enter the results of the measurements by the eddy current sensor into the display circuit that is connected to the eddy current sensor via a calculating circuit where the scopes of the phase difference and the amplitude difference are shown as a circle for the criterion for acceptance where a random variable (vector) X is μ-ησ<X<μ+ησ (wherein μ: average value, σ: standard variation, n: 2.5-3.5) and - wherein the product is determined to be acceptable if all of the plurality of points, each corresponding to the phase difference and amplitude difference of the part that was treated, are plotted outside the circle for the criterion. measuring the product with a non-destructive inspection using an eddy current sensor prior to a first SP treatment; the first SP treatment that removes a compound layer that was produced by the heat hardening; and a second SP treatment that gives compressive residual stress to a first-SP-treated surface that was treated by the first SP treatment; wherein the second SP treatment is carried out only on the products that have been proved to be acceptable by a non-destructive inspection and show that the compound layers have been removed from the first-SP-treated surface, wherein whether the products have been proved to be acceptable by the non-destructive inspection and show that the compound layers have been removed from the first-SP-treated surface is determined by the nondestructive inspection using the eddy current sensor, wherein the product is determined to be acceptable by the following: - first to measure both the phase difference (time) and the amplitude difference (voltage) of the current generated from the flux of alternating current at a plurality of points of the non-treated and treated work to be treated and - to enter the results of the measurements by the eddy current sensor into the display circuit that is connected to the eddy current sensor via a calculating circuit where the scopes of the phase difference and the amplitude difference are shown as a circle for the criterion for acceptance where a random variable (vector) X is μ-ησ<X<μ+ησ (wherein μ: average value, σ: standard variation, n: 2.5-3.5) and - wherein the product is determined to be acceptable if all of the plurality of points, each corresponding to the phase difference and amplitude difference of the part that was treated, are plotted outside the circle for the criterion. - first to measure both the phase difference (time) and the amplitude difference (voltage) of the current generated from the flux of alternating current at a plurality of points of the non-treated and treated work to be treated and - to enter the results of the measurements by the eddy current sensor into the display circuit that is connected to the eddy current sensor via a calculating circuit where the scopes of the phase difference and the amplitude difference are shown as a circle for the criterion for acceptance where a random variable (vector) X is μ-ησ<X<μ+ησ (wherein μ: average value, σ: standard variation, n: 2.5-3.5) and - wherein the product is determined to be acceptable if all of the plurality of points, each corresponding to the phase difference and amplitude difference of the part that was treated, are plotted outside the circle for the criterion.