A thermally assisted magnetic head including a slider and a light source-unit. The slider includes a slider substrate and a magnetic head part. The light source-unit includes a laser diode and a sub-mount. The magnetic head part includes a medium-opposing surface, a light source-opposing surface and a waveguide which guides laser light from the light source-opposing surface to the medium-opposing surface. The thermally assisted magnetic head includes an optimal-structure which the following optimizing conditional expression, concerning an inlet-optical path length L1 of an inlet-interval of the waveguide, and an outlet-optical path length L2 of an outlet-interval, is satisfied, m1×L1=L2 (m1 is a natural number).