The imaging properties of a fluorescent scanning optical microscope (SOM) with a slit detector are investigated by calculating the 3-D optical transfer function (3-D OTF). The bandwidth of the 3-D OTF in the lateral direction varies with the slit direction. The widest bandwidth corresponds to the confocal SOM with an extremely small pinhole detector. The widest longitudinal bandwidth also corresponds to the longitudinal one for the confocal SOM. The contrast for the microscope with the slit is lower than that for the confocal SOM. When the slit width increases, the imaging properties approach those of the conventional optical microscope.