The present invention relates to a device for adjusting an optical magnification of a microscope, the device including: a magnification adjuster adapted to change the optical magnification of the microscope in accordance with the relative rotation position with respect to a tube unit; a membrane potentiometer positioned on the rotating passage of the magnification adjuster to output an electrical signal corresponding to the contacted position of the magnification adjuster therewith; and a controller adapted to receive the electrical signal outputted from the membrane potentiometer to output the optical magnification corresponding to the rotated position of the magnification adjuster.