1. An image processing apparatus which executes image processing for a radiographical image obtained by converting, into an electrical signal, an intensity distribution of radiation that is radiated to an object and has passed through the object, comprising: defining means for defining a gray level conversion curve to be used for gray level conversion on the basis of a contrast of the image after gray level conversion of the radiographical image; and gray level conversion means for converting a gray level of the radiographical image by using the gray level conversion curve defined by said defining means.