An alkali niobate-based piezoelectric/electrostrictive ceramics sintered body including, as a main crystal phase, a perovskite type oxide containing at least one type of element selected from the group consisting of Li, Na and K as A site constituent elements and at least one type of element selected from the group consisting of Nb and Ta as B site constituent elements. The number of lattice-strained layers of the piezoelectric/electrostrictive ceramics sintered body is preferably small. A diffuse scattering intensity ratio, which is a ratio of an intensity of diffuse scattering by a lattice-strained layer present near a domain wall to a sum of an X-ray diffraction intensity of a first lattice plane and that of a second lattice plane different in interplanar spacing from the first lattice plane due to crystallographic symmetry reduction is preferably 0.5 or lower.