It is formed, over a supporting body made of a ceramic, a bonding layer composed of one or more material selected from the group consisting of mullite, alumina, tantalum pentoxide, titanium oxide and niobium pentoxide. Neutralized beam is irradiated onto a surface of the bonding layer to activate the surface of the bonding layer. The surface of the bonding layer and the piezoelectric single crystal substrate are bonded by direct bonding.