ce-mount crystal oscillator shown in FIG. 4A; andFIGS. 5A and 5B are a plan view and a sectional view of a sheet ceramic substrate which makes up the mounting substrate, respectively.DETAILED DESCRIPTION OF THE INVENTIONIn FIGS. 4A and 4B which show a surface-mount crystal oscillator according to a preferred embodiment of the present invention, constituent elements which are identical to elements