A catheter type laser illuminating apparatus for thermally removing an atheroma generated in a blood vessel. The illuminating apparatus comprises a catheter arranged to be inserted into a blood vessel, the catheter being equipped with an ultrasonic transducer for transmitting and receiving an ultrasonic wave to generate an electric signal corresponding to the reception ultrasonic wave. The catheter is further equipped with a laser transmission fiber for emitting the laser light. Also included in the illumination apparatus is a M-mode image producing section for producing an image representative of the dimension of the atheroma on the basis of the electric signal from the ultrasonic transducer. The image is displayed on a CRT in a corresponding relation to an illumination condition of the laser light to the atheroma which is at least one of an illumination time and an illumination power.