PURPOSE: To measure the form of a belt-like matter simply and irrespectively of its material by converting latent malformation under tension into displacement and detecting them by use of a contactless displacement measuring means fitted on the outside of a roller.

CONSTITUTION: Multiple divided rings 15 are arranged through an elastic matter 16 on a shaft 2 where both its ends are supported so that they can rotate. Contactless displacement detecting devices 9 are arranged in response to each divided ring in order to detect displacements due to the tension distributions of a measured strip S of these divided rings 15 and latent forms under the tension of the strip S are measured by the detection signals from these displacement detecting devices.