To provide a cap for a high frequency device excellent in air-tightness and electromagnetic shielding property, and to provide a package for the high frequency device and its manufacturing method.

A millimeter-wave device (not shown in the figure) is mounted on the upper surface of a base 2 into which an interface pin 6 is inserted. The cap 1 equipped with a hollow cap part 1a having an opening end part and a flange part 1b formed around the opening end part of the cap part is arranged so as to cover the millimeter-wave device. The inner periphery part of this flange part 1b is protruded (a convex part 3) toward the base 2 along the opening. On the outer peripheral surface of the base 2 side, a plurality of projections (a projection part 4) are formed along the periphery of the flange part 1b. The millimeter-wave device is sealed by projection welding the convex part 3 and the projection part 4 to the upper surface of the base 2.