An eyepiece base unit and a microscope with which a phase contrast observation function can be easily added to the microscope. The eyepiece base unit is removably attached to a main unit of the microscope, and includes, in a state of being attached to the microscope, a pupil conjugate plane, which is a plane conjugate with an image side focal plane of an objective lens in an observation optical system of the microscope. By rotating a turret around a central axis, phase plates installed in the turret can be inserted into the pupil conjugate plane.