1. An image processing apparatus, comprising: a sensor constituted by a plurality of elements each of which converts radiation to an electric signal; a plurality of analog-digital converters for converting the electric signals to partial image data for respective partial image data areas; a calculation unit which calculates an inclination of a line segment crossing partial image data for respective partial image data areas adjacent to each other, based on inclinations of the partial image data between an edge contacting a boundary of the respective partial image data areas and points which are predetermined distances apart from the edge in the respective partial image data areas, and then calculates a step value based on the inclination; and an image sensing means for correcting steps between the adjacent partial image data-which are output from the respective analog-digital converters, based on the step value calculated by the calculation unit to obtain an image data.