A yarn defect detecting device comprising: a yarn thickness detecting section that detects a thickness of a traveling yarn; a yarn defect determining section including a type determining section that determines a type of the yarn defect based on the yarn thickness and determines whether to measure a length of the defect depending on the defect type; wherein the yarn defect determining section determines, for defect types identified by the type determining section from the yarn thickness as neps or slubs, not to measure the length of the defect, and wherein the yarn defect determining section determines, for other defect types identified by the type determining section from the yarn thickness as being thin places or thick places, whether a part of the yarn detected by the yarn thickness detecting section having a thickness falling outside a thickness threshold specified as a determination threshold of such a defect of the yarn has a length equal to or larger than a reference length specified as the determination threshold and determines, when the part of the yarn having the thickness falling outside the thickness threshold specified as the determination threshold has a length equal to or larger than the reference length, the part of the yarn falling outside the thickness threshold specified as the determination threshold as a defect corresponding to a thin place or a thick place and determines to measure the defect length thereof; and the yarn defect detecting device further comprises an end point identifying section that identifies an end point of the defect in a longitudinal direction when the part of the yarn is determined as the defect corresponding to a thin place or thick place by the yarn defect determining section. a yarn thickness detecting section that detects a thickness of a traveling yarn; a yarn defect determining section including a type determining section that determines a type of the yarn defect based on the yarn thickness and determines whether to measure a length of the defect depending on the defect type; wherein the yarn defect determining section determines, for defect types identified by the type determining section from the yarn thickness as neps or slubs, not to measure the length of the defect, and wherein the yarn defect determining section determines, for other defect types identified by the type determining section from the yarn thickness as being thin places or thick places, whether a part of the yarn detected by the yarn thickness detecting section having a thickness falling outside a thickness threshold specified as a determination threshold of such a defect of the yarn has a length equal to or larger than a reference length specified as the determination threshold and determines, when the part of the yarn having the thickness falling outside the thickness threshold specified as the determination threshold has a length equal to or larger than the reference length, the part of the yarn falling outside the thickness threshold specified as the determination threshold as a defect corresponding to a thin place or a thick place and determines to measure the defect length thereof; and the yarn defect detecting device further comprises an end point identifying section that identifies an end point of the defect in a longitudinal direction when the part of the yarn is determined as the defect corresponding to a thin place or thick place by the yarn defect determining section.