Abstract: This paper discusses ultrasonic mechanism of the sealed-H₂ release from basalt rock on the basis of experiments.A small amount of released H₂ by ultrasonic vibration is escaping H₂,whereas most H₂ is absorbed H₂.Both phenomena can be regarded as a precursory H₂.So,it is possible to use H₂.to.forcast earthquake.Seeing that the intensity of u.v.and the its duration were constant in the experiments and the amount of released H₂ gas increased during each vibration,abnormal H₂ seems not to be representative of accoustic emission intensity,thus suggesting that the intensity of the failure of rock may be representative of the earthquake magnitude.