To provide a Poisson's ratio measuring method and its device with high accuracy even to a sample to be measured in the shape of an elongated rod.

An ultrasonic oscillating means 42 is connected to a signal generator 44 via an amplifier 45 and is connected to a signal analyzer 47 via an amplifier 46. Therefore, a tone burst signal generated at the signal generator 44 is amplified at the amplifier 45 to drive the ultrasonic oscillating means 42. By this, elastic waves are transmitted through a specimen rod 41, and the waves reflected at the end face on the opposite side are detected by the ultrasonic oscillating means 43. The analyzing means 47 compares the transmitted elastic waves with the received elastic waves to obtain Poisson's ratio from the amount of attenuation of the elastic waves.