PURPOSE:To make it possible to perform measurement even with respect to a flexible article in a non-contact state, by irradiating the side surface of a cylindrical article to be measured with slit light and observing the positional variation of the slit light generated when the article to be measured is rotated by a microscope arranged in an inclined state. CONSTITUTION:A cylindrical article 3 to be measured is mounted on a rotary mount stand 1 and slit light is allowed to irradiate the peripheral surface of said article 3 to be measured from the lateral direction to form a linear light mark 7. A microscope-moving mechanism 11 is provided in the vicinity of the rotary mount stand 1 and a contour-detecting microscope 8 is arranged to said mechanism 11 so as to be properly inclined with respect to a vertical direction. The, the rotary mount stand 1 is slowly rotated and the positional variation of the linear light mark 7 in the visual field of the microscope 8 at the time of rotation is observed and the out-of-roundness of the article 3 to be measured is measured from said variation quantity and the angle of rotation of the mount stand 1. By this method, because measurement can be performed in a non- contact state, a flexible article or a small light wt. article can be also measured.
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