In a method for in vivo diagnostics of a biological tissue covered with epithelium an image of the biological tissue is acquired with the aid of a beam in the visible or near IR range directed towards a biological tissue by visualizing the intensity of optical radiation backscattered by the biological tissue. The basal membrane of said biological tissue, which separates the epithelium from an underlying stroma, is identified in the acquired image and diagnostics is performed on basis of the form of the basal membrane. For diagnostics of biological tissue lining the surface of cavities and internal organs of a patient a miniature optical fiber probe is inserted into the patient's cavity. The probe may be placed at a distal end of an endoscope instrumental channel. Acquired images show that a biological tissue covered with healthy epithelium has a smooth basal membrane, which separates stratified squamous epithelium from underlying connective tissue, while pathological regions of biological tissue are characterized by a change in the shape of the basal membrane, or violation of its integrity, or its absolute destruction. Using low coherent optical radiation for implementing the developed method ensures high spatial in-depth resolution.