PURPOSE: To avoid the lowering in the oscillation frequency, by preventing the lowering in the degree of vacuum through the complete sealing of the mating part between the stem and the case, by bonding the material itself molten constituting the stem and the case.

CONSTITUTION: At the bonding between the stem 2 of Kovar mounting the crystal oscillator 1 and the sealing case 4 of Kovar, electron beam 7 is emitted to directly melt and bond the Kovar itself constituting the stem and the case. Thus, even if there is any clearance due to dispersion of shape and size of the mating part between the case and the stem, perfect sealing is made by molten material. Further, since soldering plating or In-Sn plating which is a factor of gas production is not used to the sealing part at all, no lowering in vacuum in the case is caused to prevent the oscillating frequency of the crystal oscillator from being lowered.