In an image processing apparatus of the invention, HVC conversion is performed stably. THat is, an image sensor reads image data of red, green and blue of a document, while a memory stores brightness data V of a color patch. When the image data are converted to a brightness data V and color difference data WR and WB, coefficients used in the conversion of the HVC conversion are calculated by an operator based on the image data of red, green and blue of the color patch read by the image sensor and the brightness data V stored in the memory.