Provided is a thermally-assisted magnetic recording head in which a slider including an optical system is joined with a light source unit. The light source unit comprises: a unit substrate including a joining surface joined with the slider and a source-installation surface adjacent to the joining surface; a light source provided in the source-installation surface and emits light for thermal assist; and a photodetector section formed inside the unit substrate, a light-receiving portion of the photodetector section for receiving light emitted from a rear light-emission center being located on the source-installation surface side. The light source unit includes the photodetector section that enables constant monitoring of light output from the light source. Accordingly, feedback adjustment of the light output can be accomplished. Further, since the rear light-emission center and the light-receiving portion can be located sufficiently close to each other, the light output can be monitored with a higher efficiency.