1. A non-transitory computer readable medium having a program including a set of instructions for permitting a computer to perform volume rendering on volume data and display an image on a display screen, said set of instructions comprising: upon changing a scale factor and/or a viewpoint direction, displaying the image wherein different portions of the displayed image are one-at-a-time sequentially volume rendered such that respective resolutions of the different portions of the displayed image are one-at-a-time automatically sequentially increased over time from an initial image resolution, wherein a display of a first portion with increased resolution is maintained while at least a second portion that is outside of the first portion in the displayed image, and that surrounds the first portion in the displayed image, is volume rendered then displayed with increased resolution, resulting in an overall resolution of the displayed image automatically increasing over time from the initial image resolution, wherein the step of displaying comprises the steps of: (a) executing a first image generating process which generates a first image of a first two-dimensional region of the displayed image, requiring a first calculation amount for a first three-dimensional region of the volume data that corresponds to the first two-dimensional region, wherein the volume data includes a plurality of voxels, and wherein said first two-dimensional region is said first portion; (b) automatically executing a second image generating process which is different from said first image generating process and generates a second image requiring a second calculation amount for said first three-dimensional region, wherein said second calculation amount is larger than said first calculation amount; (c) displaying sequentially each of the first and second images in the displayed image such that a resolution of the first two-dimensional region is sequentially increased from the initial image resolution to a higher resolution due to the second image replacing the first image in the displayed image; (d) while the second image is displayed in the displayed image, automatically executing a third image generating process which is different from said first image generating process and generates a third image requiring a third calculation amount for a second three-dimensional region of the volume data, said second three-dimensional region corresponding to a second two-dimensional region of the displayed image, wherein said third calculation amount is larger than said first calculation amount, and wherein said second two-dimensional region is said second portion and said third image is outside of and surrounds said first and second images; and (e) displaying the third image in the displayed image with the second image.