This paper describes a method for measuring elastic constants, sound velocity and their changes,especially very small changes,newly devised by the authors. The principle of measuring method is based on the use of an electrostatic-type mechanical vibrator-controlled oscillator. The new oscillator is capable of measuring elastic constants and its small changes of the order of 10−6 or smaller precisely by comparing the oscillation frequency of the oscillator with a known accurate standard frequency (e.g., the standard frequency broadcasted by JJY). As examples, the results of measurement of Young’s modulus of some metals and of very small anomalous change of resonant frequency (or sound velocity) of specimen vibrator made of co-elinvar alloy are described.