A thermally-assisted magnetic head includes a slider including an air bearing surface (ABS) and a waveguide with a core that extends from a light entering surface to the ABS, and a laser diode (LD) unit attached to the light entering surface. The LD unit faces the light entering surface, and a photo detector faces the ABS. A polarizer that only transmits light having a polarization component orthogonal to a polarization direction of linearly polarized laser light is disposed between the ABS and the photo detector. An LD of the LD unit is activated, and the linearly polarized laser light enters the core from the light entering surface. Light radiated from the ABS enters the polarizer, and an alignment of the slider and the LD unit is performed while the photo detector detects the light transmitted through the polarizer.