The resolution of a fluorescent confocal scanning optical microscope (CSOM) is superior to that of a conventional fluorescent optical microscope. To attain this superiority, the fluorescent CSOM uses a pinhole in front of the detector. Thus, the resolution of the CSOM is dependent on the pinhole radius. Three-dimensional optical transfer functions are calculated for the various radii to elucidate this dependence. The results show that a CSOM with a radius smaller than ~1 optical unit has a bandwidth comparable with that of an infinitely small radius.