Abstract: A safety device for detecting troubles in an optical transmission fiber of a medical laser application to prevent accidents characterized by using either a laser light reflector from the exit end surface of the fiber or a secondary light. When trouble is detected, the safety device operates a mechanism for interrupting the coupling of a beam of laser light into the transmission fiber. The safety device includes an electrical circuit which allows appropriate use of the apparatus with the tip of the fiber in water or a physiological salt solution for treatment of a tumor in a urinary bladder. The electrical circuits will include amplifiers, comparators and logic circuits.