A laser scanning microscope is disclosed that combines light from first and second scanning optical systems onto a common optical path using a beam combiner. A first plane-parallel transparent plate and a beam combiner are positioned on a support that may be moved relative to the common optical path. The first plane-parallel transparent plate has an optical thickness equal to the optical thickness of the beam combiner and is oriented with its surface normal lying substantially parallel with the surface normal of the beam combiner. An astigmatism-correcting optical element is provided on the common optical path, with the astigmatism-correcting optical element generating astigmatism that is equal in magnitude, but different in direction by 90 degrees, to the astigmatism generated by non-collimated light that is transmitted through the beam combiner. When the support is repositioned, spherical aberration in various light paths is maintained constant so that it may be easily corrected.