1. An image processing method in an image processing apparatus for converting a gradation image into vector data, the method comprising: a color change direction detecting step of detecting color change directions of pixels in the gradation image; a gradation type determining step of determining gradation type of the gradation image as linear gradation when the color change directions detected in the color change direction detecting step agree with an identical direction; and a description step of describing the gradation image using a graphic language in accordance with the determined gradation type.