Simple pseudo gradation conversion technique suitable for implementation in portable devices such as mobile phones employing a relatively low-speed processor. Target image is represented with 2.sup.Cn gradation levels for n kinds of color components (where Cn is a natural number for each color component, and n is a natural number) per pixel. Image information acquisition unit (51) acquires image information representing an original image subjected to conversion from camera front end (40) and modulation/demodulation unit (61). Color space conversion unit 55 converts luminance color difference information, if the original image is represented thereby, into color component information. Quantization unit (54) quantizes the color component information into (2.sup.(Cn+1)-1) gradation values for each color component. Color determination unit (56) determines one-half of the quantized value as a target value, if the quantized value is an even number, and if an even number, one of the two integers preceding and following that half value is determined as a target value according to a pixel position so that those integers are arranged checkerwise.