PURPOSE: To measure the minute displacement readily, by fixing a wedge shaped magnetic member to a material to be measured, and detecting the displacement of the inclined surface of said member.

CONSTITUTION: A wedge shaped detecting body 10 which comprises a magnetic member having constant permeability and has a vertical angle θ is fixed to the material to be measured M as a unitary body. The slant direction of the inclined surface 15 of the detecting body 10 is aligned with the vibrating direction of the material to be measured M. A detecting head 11, which detects the variation in electromagnetic energy due to a magnetic body, such as a magnetic sensor and an overcurrent detector is provided in front of the inclined surface 15. Since a distance (d) between the detecting head and the detecting body is changed in response to the vibrating displacement (x) of the material to be measured, the detected output also changes. The result is displayed on an pen recorder 12.