1. An image processing apparatus comprising: a selected-generated-code-size computing section that codes an input image by at least quantizing simple coded data that is obtained by simple coding of the input image through simple processing, by using quantization steps based on selected quantization factors discretely selected from quantization factors, and computes generated code sizes of the input image upon coding; an error correcting section that corrects an error in the generated code sizes of the input image upon coding which occurs in accordance with the simple processing; and an interpolated-generated-code-size computing section that computes generated code sizes when the input image is coded on the basis of quantization parameters other than the selected quantization parameters, by an interpolation process with respect to the generated code sizes when the input image is coded on the basis of the selected quantization factors.