An otoscope comprises a guide tube which is to be inserted into the external auditory meatus of a patient and a side-viewing microscope which is reciprocated through the guide tube. A light beam direction-diverting device is provided in the distal end section of the guide tube. Where the microscope is in a retracted position, the light beam direction-diverting device faces an illumination window and observation window in the lateral wall of the distal end section of the microscope to divert light beams emitted from the illumination window ahead of the microscope in its axial direction and light beams conducted to the guide tube along the microscope toward the observation window. The microscope protrudes from the proximal end of the guide tube. When, therefore, the proximal end of the microscope is pushed into the guide tube, the distal end of the microscope protrudes from the distal end of the guide tube. A microscope-actuating device is provided between the guide tube and microscope. This microscope-actuating device normally locks the microscope in a retracted position. When manually operated, the microscope-actuating device lets the microscope protrude from the distal end of the guide tube. Mounted on the distal end of the guide tube is a protective ring which contacts the outer periphery of the external auditory meatus. Therefore, the protective ring ensures the insertion of the microscope into the middle ear and the protection of the inner wall of the middle ear, and further prevents a small hole previously opened in the eardrum from being unnecessarily enlarged.