A device for measuring sound velocity in material comprises a piezoelectric substrate, first- and second comb-shaped electrodes, a counter electrode, and a reflector. When input electric signals with a frequency f, respectively, are applied between the first comb-shaped electrode and the counter electrode in turn, longitudinal waves are radiated into a material existing between the counter electrode and the reflector, and then, reflected at the reflector, and after all, detected between the second comb-shaped electrode and the counter electrode as delayed electric signals Di in accordance with distances Zi between the counter electrode and the reflector. Electrical coupled-signals from the input electric signals and the delayed electric signals Di interfere respectively, so that respective interference signals Ri occur. A sound velocity V in the material is estimated from the frequency f and the interference signals Ri.