An side irradiating type laser ray irradiation apparatus for irradiating a tissue with a laser ray having a deep transmitting capability for the purpose of treating, for example, Benign Prostatic Hyperplasia, cancer or other tumors. The apparatus includes an irradiating unit for reflecting the laser ray, a transporting device for transporting the irradiating unit, and an interlocking device for changing the irradiation angle of the laser ray in correspondence with the movement so that the laser ray radiated from the moving irradiating unit always passes through the same point. Since the laser ray constantly passes through a point in a deep area of the tissue, it is capable of effectively heating only the deep lesional region.