1. An image processing device executing image processing on volume data of a 3-dimensional image, the image processing device comprising: an extraction process executing unit that extracts a 3-dimensional initial region satisfying a condition given in advance from the volume data; a region correction process executing unit that extracts a 3-dimensional corrected region by performing a correction process on the initial region; and a visualization process executing unit that generates a plurality of cross-sectional diagrams of the 3-dimensional image from the volume data and outputs at least some of the plurality of cross-sectional diagrams, wherein when the initial region is displayed, the visualization process executing unit selects one or more cross-sectional diagrams having voxels included in the initial region from the plurality of cross-sectional diagrams generated by the visualization process executing unit, and outputs the selected one or more cross-sectional diagrams so that the voxels included in the initial region are distinguishable from other regions, when the corrected region is displayed, the visualization process executing unit selects one or more cross-sectional diagrams having voxels included in the corrected region from the plurality of cross-sectional diagrams generated by the visualization process executing unit, and outputs the selected one or more cross-sectional diagrams so that the voxels included in the corrected region are distinguishable from other regions, and wherein, when the corrected region is displayed after the initial region has been displayed and a first number of cross-sectional diagrams having voxels included in the initial region is different from a second number of cross-sectional diagrams having voxels included in the corrected region, the visualization process executing unit changes a number of cross-sectional diagrams to be output from the first number to the second number.