e surface-mount crystal oscillator shown in FIG. 1;FIGS. 3A and 3B are a plan view and a sectional view of the sheet ceramic substrate which makes up a mounting substrate, respectively;FIG. 4A is an exploded sectional view showing the assembly of a surface-mount crystal oscillator according to an embodiment of the present invention;FIG. 4B is a plan view of the mounting substrate used in the surfa