To provide a microscope equipped with a focus detecting means whose focusing detectable range is wide and where the shape of a focus error signal is symmetric on the positive side and the negative side around a focal position as the center.

In this microscope equipped with a differential beam size system focus detecting means, a 1st detection part 18 of the focus detecting means is arranged at a position separate from a condensing optical system 16 by a distance L1 from a focal point 20 of the optical system 16 and a 2nd detection part 19 is arranged at a position near to the optical system 16 by a distance L2 from a focal point 21 of the optical system 16. At this time, the L1 and the L2 are set to be L1≠L2 by coinciding with the asymmetric displacement of the converging point of a luminous flux by the optical system 16.