A computed tomography (CT) image processing apparatus includes an image processor which sets two or more CT number ranges of interest defined by a window level and a window width for CT numbers of CT image data, and maps the CT numbers to display grayscale values of a display. The display displays the CT image data according to a mapping result. A gradient of a graph showing a relationship between the CT numbers and the display grayscale values in a CT number range included in the two or more CT number ranges of interest, is greater than a gradient in a CT number range not included in the two or more CT number ranges of interest. The graph has a zero or positive gradient over an entire section, or has a zero or negative gradient over the entire section.