1. An image correction apparatus, comprising: an image intake unit that obtains image data from outside; a histogram preparation unit that prepares a histogram by calculating a pixel number ratio at each gradation against the total number of pixels based upon the image data; and a gradation correction unit that corrects a gradation width in said histogram based upon a comparison of a threshold value for said pixel number ratio at a gradation and the pixel number ratio at each gradation in said histogram, said threshold value being determined through user specification.