PURPOSE: To make it possible to measure the amount of undulation of a tape in the direction of width readily and quickly, by providing a straight reference body, which is streched by tension in the longitudinal direction and includes a wire member, whose one end in the direction of a diameter is made to be a straight edge.

CONSTITUTION: A measuring device is stretched by tension in the longitudinal direction and provided with a straight reference body 1 comprising a wire member 1A with a circular cross section, whose one end in the direction of a diameter is made to be straight edge E. Both ends of the straight reference body are stretched on supporting members 6A and 6B. One end of the body exceeding the supporting member 6A is pulled downward by the tensile strength of a weight 7. The other end exceeding the supporting member 6B is supported by a pulling spring 8 against the tensile strength of the weight 7. Thus the constant tensile strength is applied to the wire member 1A. Therefore, the amount of undulation of the tape in the direction of the width can be readily and quickly measured.