Two groups of inclined lines, which are included in a spurious wave scattering means, are formed at opposite angles with respect to each other in the vicinity of an upper edge of a substrate. The angles of the inclined lines are such that they are close to perpendicular toward the central portion of the substrate, and gradually decrease toward the edges thereof. In a similar manner, two other groups inclined lines, which are also included in the spurious wave scattering means, are formed at opposite angles with respect to each other, with gradually changing angles. The spurious waves that reach these regions are diffuse by the inclined lines, so that they are not propagated to converters (sensors). Three rectangular spurious wave scattering means, formed by inclined lines, inclined at angles other than 45°, also function to diffuse and eliminate spurious waves that propagate along the front surface of the substrate.