This invention provides a method and a system for preventing charring at a laser beam emitting site during treatment or diagnosis using a laser catheter for applying a laser beam. The method is intended to control laser beam irradiation of an apparatus equipped with a laser catheter comprising a laser beam transmission means and a laser beam emitting site used for diagnosis or treatment with the irradiation of the inside of a blood vessel or heart cavity with a laser beam. The method for controlling laser beam irradiation is intended to prevent blood charring at a laser emission site of an apparatus equipped with a laser catheter, and the method comprises a step of controlling a laser beam output in accordance with temporal changes in the intensity of the diffuse reflected light beam by erythrocytes applied to the inside of a blood vessel or heart cavity.
