In the case where an ultraminiature piezoelectric substrate, which has a resonating portion formed by making a concavity by etching in the surface of the piezoelectric substrate made of an anisotropic crystal material, is mass-produced by batch operation using a large-area piezoelectric substrate wafer, an annular portion surrounding each concavity is formed sufficiently thick to prevent cracking from occurring when the wafer is severed. A piezoelectric substrate 2 of an anisotropic piezoelectric crystal material has a thin resonating portion 4 and a thick annular portion 5 integrally surrounding the outer marginal edge of the resonating portion to form a concavity 3 in at least one of major surfaces of the substrate; the inner wall 5a of the annular portion in the one crystal orientation slopes gently more than the inner wall in the other crystal orientation perpendicular to said one crystal orientation, and the piezoelectric substrate is longer in said one crystal orientation than in the other crystal orientation.