A second waveguide is formed near a first waveguide for guiding light to the vicinity of a main pole of a thermally assisted magnetic recording head, and a portion of light propagated through the waveguide 1 is branched to the second waveguide. The light transmitting in the second waveguide is detected by a photodetector to detect an intensity of the light propagated through the first waveguide. In the magnetic recording apparatus, an intensity of a semiconductor laser is decreased when an amount of light incident to the photodetector is large and the intensity of the semiconductor laser is increased when the amount of light incident to the photodetector is small. By constituting a feedback loop as described above, the intensity of the light propagated through the first waveguide is kept constant.