In the measurement method, a vibrator (5) of a detection unit (4) is relatively moved against a plate (3) of which surface is flat having predetermined area. The vibrator (5) which is vibrating is moved closer to a surface of an object-to-be-measured (1) which is mounted on the plate (3) until the frequency thereof is varied. A position where the frequency of the vibrator (5) is varied is output as a contact position where the vibrator (5) is contacted to the object-to-be-measured (1). Then, thickness of the object-to-be-measured (1) is measured by comparing the position where the vibrator (5) is contacted to the object-to-be-measured (1) with a surface position of the plate (3).