Recently, wide-gamut monitors and printers have begun to appear in the market. However, there are no appropriate tools for evaluating color reproduction in such image output devices. This report describes a method to create the image data for evaluation of the wide-gamut devices. The method expands the gamut of the source image in a manner that a color in the image is mapped into a color with higher chroma and the same hue and lightness, by using a chroma mapping function. The method was applied to sRGB images such as XYZ-SCID images to make the test images for evaluating wide-gamut output devices. In order to find the optimal chroma mapping function, a psychophysical experiment was performed, where the test images made with three types of mapping functions were visually evaluated. Results show that observers judged the test images expanded with nonlinear functions were the best quality.