Color facsimile communication uses image data obtained by JPEG-compressing data having a CIE Lab color space. To attach a JPEG image to e-mail, image data obtained by JPEG-compressing data having an Internet standard YCbCr color space is desirable. When an image transmission method is selected, color space conversion (conversion to a CIE Lab or YCbCr color space) corresponding to the selected transmission method (facsimile transmission or transmission by attaching to e-mail) is performed for an image to be transmitted. This image subjected to the color space conversion is encoded by JPEG, and the encoded image is transmitted by the selected transmission method.