An image processing method of conducting gradation processing on a radiation image having signal values according to an irradiation amount of a radiation ray transmitting through a subject by converting the signal values with a lookup table to obtain a suitable image for diagnosis using the radiation image, the image processing method comprising: a feature amount calculating step of calculating a feature amount of the radiation image data by a feature amount calculating means, the feature amount being an absolute value of an edge component; a feature amount integrating step of integrating the feature amount of radiation image data by a feature amount integrating means; a feature amount evaluating step of evaluating the feature amount calculated by the feature amount calculating means and integrated by the feature amount integration means by means of a feature amount evaluating function E(S, G); a parameter determining step of determining parameters, namely an S value and a G value, wherein a parameter determining means obtains the S value and the G value by making the results of the evaluation conducted by feature amount evaluating means with the feature amount evaluating function E(S, G) to be the maximum such that the feature amount evaluating function E(S, G) is maximized, wherein E(S, G) represents the evaluating function to evaluate how an amplitude of a value has been changed after passing the look-up table at the edge points exceeding a certain threshold value, wherein the following expression is made to hold: E S, G = EDGE S, G / ORG _ E S, G, wherein EDGE(S, G) is average contrast of the image after look-up table conversion, and ORG_E(S, G) is average contrast of the original image, wherein the feature amount conversion is conducted by a look-up table LUT(Gx-S) with certain S, G values as variables shift value S and gradation value G of the look-up table that determines gradation characteristics to obtain the ratio of edges; and an image processing step of conducting gradation processing with the lookup table LUT(Gx-S) defined by the parameter S and G determined in the parameter determining step. a feature amount calculating step of calculating a feature amount of the radiation image data by a feature amount calculating means, the feature amount being an absolute value of an edge component; a feature amount integrating step of integrating the feature amount of radiation image data by a feature amount integrating means; a feature amount evaluating step of evaluating the feature amount calculated by the feature amount calculating means and integrated by the feature amount integration means by means of a feature amount evaluating function E(S, G); a parameter determining step of determining parameters, namely an S value and a G value, wherein a parameter determining means obtains the S value and the G value by making the results of the evaluation conducted by feature amount evaluating means with the feature amount evaluating function E(S, G) to be the maximum such that the feature amount evaluating function E(S, G) is maximized, wherein E(S, G) represents the evaluating function to evaluate how an amplitude of a value has been changed after passing the look-up table at the edge points exceeding a certain threshold value, wherein the following expression is made to hold: E S, G = EDGE S, G / ORG _ E S, G, wherein EDGE(S, G) is average contrast of the image after look-up table conversion, and ORG_E(S, G) is average contrast of the original image, wherein the feature amount conversion is conducted by a look-up table LUT(Gx-S) with certain S, G values as variables shift value S and gradation value G of the look-up table that determines gradation characteristics to obtain the ratio of edges; and an image processing step of conducting gradation processing with the lookup table LUT(Gx-S) defined by the parameter S and G determined in the parameter determining step.