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 slider substrate includes a light source-cavity formed in a light source-placing surface on which the light source-unit is placed. The light source-cavity includes an opening concave part being formed larger than a mount bottom surface of the sub-mount. The mount bottom surface of the sub-mount is inserted into the opening concave part to be joined to the light source-cavity.