Abstract: A self-straightening tape measure which is commonly called a convex rule, has hitherto been prepared exclusively of steel, however, the tape measure of steel is not only liable to be rusted but also is dangerous owing to good conductor of electricity when used in the vicinity of electric wires. In order to eliminate the above disadvantages, the present invention provides a self-straightening tape measure of resin, which is prepared by impregnating a tape material of crossing fibers with a large amount of thermosetting resin, bending the tape material in the width direction to form an arc in cross section, so that the arc may occupy from one-third to three-fourths of the whole circumference of the circle described by extending the arc, and heating the tape material to harden the resin and to have the said arcuate form.