1. A confocal scanning microscope, comprising: a projected image building unit building an image, which is projected on three planes mutually intersecting in three-dimensional space, of a three-dimensional image obtained by stacking in a direction of an optical axis a plurality of two-dimensional confocal images of a sample, which are obtained by changing a focal position of an objective lens against the sample to be observed in the direction of the optical axis of the objective lens; and a target region forming unit forming a target region implemented by integrating pixels, whose points projected on the three planes are included in regions respectively specified for the three planes, among pixels configuring each of the two-dimensional confocal image.