An object of the present invention is to provide a bonded substrate which is excellent in temperature characteristics and suppresses unnecessary response due to reflection of an elastic wave at a bonding interface.
[Means to Solve the Problems] The present invention is unique in that a bonded substrate is constructed by bonding a LiTaO3 substrate and a base plate wherein a Li concentration at a base plate-bonding face of the LiTaO3 substrate is higher than that at a LiTaO3 substrate-side end face of the bonded substrate, that the difference between the Li concentration at the base plate-bonding face of the LiTaO3 substrate and the Li concentration at the LiTaO3 substrate-side end face of the bonded substrate is 0.1 mol % or greater, that the Li concentration at the base plate-bonding face of the LiTaO3 substrate satisfies an equation Li/(Li+Ta)×100=(50+α) mol %, where α is in the range of −1.2