PURPOSE: To fix a resonator so as not to be damaged by opposing a short-circuit face of the resonator to a case inner face with a gap and locating a U-shaped spring member in this gap.

CONSTITUTION: The resonator 1 is put in a concaved part 9 while the resonator 1 is supported horizontally. Then an open face of the resonator 1 is touched lightly to a boundary face 14 of the part 9. The U-shaped metal or a plastic- made spring member 16 is pushed into the gap 15 formed between the short- circuit face of the resonator 1 and an inner wall of the case 8 from the upper direction. Then the resonator 1 is depressed axially, brought firmly into contact with the boundary face 14 of the part 9 and fixed.