1. A confocal scanning microscope, comprising: a projected image building unit which builds projected two-dimensional images on respective planes mutually intersecting in three-dimensional space from a three-dimensional image projected on the mutually intersecting planes, the projected three-dimensional image being obtained by stacking a plurality of two-dimensional confocal images of a sample in a direction of an optical axis of an objective lens, the plurality of two-dimensional confocal images being obtained by changing a focal position of the objective lens against the sample in the direction of the optical axis of the objective lens; a specified region obtaining unit which obtains two-dimensional regions specified for the projected two-dimensional images on each of the three planes, and which represents the specified two-dimensional regions on the respective projected two-dimensional images; a target region forming unit which forms a three-dimensional target region from the specified two-dimensional regions of the projected two-dimensional images on the three planes; and a target region display unit which displays an image where the three-dimensional target region is represented along with the projected three-dimensional image.