This disclosure relates to a digital holographic microscope which is robust to an external environment. According to an aspect of the present embodiment, there is provided an optical system for a digital holographic microscope, the optical system including a beam splitter which reflects a light radiated from a light source toward an object, or passes a light reflected from and traveling from the object; an objective lens focusing the light reflected by the beam splitter on the object; a transflective mirror which is located on the surface of the objective lens and is determined to be transparent or reflective depending on a polarization direction of a light incident to the transflective mirror; and a wave plate which converts the light passing through the beam splitter into a circularly polarized light.