Embodiments of the present invention help to provide a magnetic disk drive in which the base and the lid are securely welded together, so that the low-density gas sealed therein is protected from leakage. According to one embodiment, the peripheral surface of the rib of the base is machined to form the surface, which is free of electrocoating and weld surface. The outer lid, which completely conforms to the contour of the base, is placed on the rib, and the laser beam is directed onto the outer lid. Thus the position of laser beam irradiation varies within only ±0.05 mm for both the base and the lid. Laser welding in this manner yields the bead invariably, which makes the enclosure completely sealed without helium leakage.