A yarn travelling information acquiring device comprising: a first detecting section adapted to detect a thickness unevenness of a travelling yarn and to output first yarn thickness unevenness signals; a second detecting section arranged upstream in a yarn travelling direction at a distance from the first detecting section, and adapted to detect the thickness unevenness of the yarn and to output second yarn thickness unevenness signals; sampling means adapted to sample the first and second yarn thickness unevenness signals with a sampling speed selected to maintain the number of data acquired per unit length substantially constant; a similarity degree evaluating section adapted to use a first imaginary frame and a second imaginary frame to select a plurality of reference positions of the first imaginary frame on a time axis of the second imaginary frame, and to evaluate a plurality of similarity degrees of the first yarn thickness unevenness signals and the second yarn thickness unevenness signals, the first imaginary frame being formed in accordance with the first yarn thickness unevenness signals extracted from the first yarn thickness unevenness signals acquired within a first time range, and the second imaginary frame being formed in accordance with the second yarn thickness unevenness signals extracted from the second yarn thickness unevenness signals acquired within a second time range that is longer than the first time range; and a travelling information acquiring section adapted to calculate a time delay (ΔT) between the first yarn thickness unevenness signals forming the first imaginary frame and the second yarn thickness unevenness signals forming the second imaginary frame in accordance with the similarity degrees, and to acquire travelling information of the yarn in accordance with the distance and the time delay (ΔT), with a sampling speed acquiring section adapted to acquire a sampling speed of the yarn, characterized in that when the sampling speed acquired by the sampling speed acquiring section is less than a threshold, the similarity degree evaluating section is adapted to process all the first yarn thickness unevenness signals output by the first detecting section and the second yarn thickness unevenness signals output by the second detecting section extracted, and when the sampling speed acquired by the sampling speed acquiring section is equal to or greater than the threshold, the similarity degree evaluating section is adapted to process only a part of the first yarn thickness unevenness signals output by the first detecting section and of the second yarn thickness unevenness signals output by the second detecting section. a first detecting section adapted to detect a thickness unevenness of a travelling yarn and to output first yarn thickness unevenness signals; a second detecting section arranged upstream in a yarn travelling direction at a distance from the first detecting section, and adapted to detect the thickness unevenness of the yarn and to output second yarn thickness unevenness signals; sampling means adapted to sample the first and second yarn thickness unevenness signals with a sampling speed selected to maintain the number of data acquired per unit length substantially constant; a similarity degree evaluating section adapted to use a first imaginary frame and a second imaginary frame to select a plurality of reference positions of the first imaginary frame on a time axis of the second imaginary frame, and to evaluate a plurality of similarity degrees of the first yarn thickness unevenness signals and the second yarn thickness unevenness signals, the first imaginary frame being formed in accordance with the first yarn thickness unevenness signals extracted from the first yarn thickness unevenness signals acquired within a first time range, and the second imaginary frame being formed in accordance with the second yarn thickness unevenness signals extracted from the second yarn thickness unevenness signals acquired within a second time range that is longer than the first time range; and a travelling information acquiring section adapted to calculate a time delay (ΔT) between the first yarn thickness unevenness signals forming the first imaginary frame and the second yarn thickness unevenness signals forming the second imaginary frame in accordance with the similarity degrees, and to acquire travelling information of the yarn in accordance with the distance and the time delay (ΔT), with a sampling speed acquiring section adapted to acquire a sampling speed of the yarn, characterized in that when the sampling speed acquired by the sampling speed acquiring section is less than a threshold, the similarity degree evaluating section is adapted to process all the first yarn thickness unevenness signals output by the first detecting section and the second yarn thickness unevenness signals output by the second detecting section extracted, and when the sampling speed acquired by the sampling speed acquiring section is equal to or greater than the threshold, the similarity degree evaluating section is adapted to process only a part of the first yarn thickness unevenness signals output by the first detecting section and of the second yarn thickness unevenness signals output by the second detecting section.