An organic silicon compound 10 is applied around vibrating electrodes 3a and 3b of a piezoelectric resonant element 1. A compound such as silane, chlorosilane, silazane, silthiane, siloxane, cyclosilane, cyclosilazane, cyclosilthiane, cyclosiloxane, silanol, or metallosilicone is used as the organic silicon compound 10. Around the piezoelectric resonant element 1 and the organic silicon compound 10, for example, an ultraviolet ray curing resin is applied, and the ultraviolet ray curing resin is cured, thereby a permeable film 11 is formed. A cavity 12 is formed around the vibrating electrodes 3a and 3b by dispersing the organic silicon compound 10 to the outside through the film 11. Around the film 11, for example, an outer coating resin is applied, and the outer coating resin is cured, thereby an outer coating material 13 is formed.