A rotation detector includes a semiconductor chip, a nonmagnetic case body, a biasing permanent magnet and a nonmagnetic cap fixed to the case body, wherein the case body has a joint surface, and the cap has an inside surface in contact with the joint surface. A method of manufacturing the rotation detector includes the following steps: assembling the semiconductor chip, the case body and the biasing permanent magnet into a unit; forming at least one groove adjacent to the joint surface so as to form a space between the joint surface and the inside surface when the cap is fixed to the case body; fixing the cap to the case body so that the inside surface of the cap can be in contact with the joint surface; irradiating a laser beam at portions behind the inside surface of the cap under a prescribed pressure to melt materials of the case body and the cap and to make melted materials flow to the space; and cooling the melted materials to re-crystallize.