A yarn monitoring device configured to monitor yarn traveling, the yarn monitoring device comprising: a discrimination section configured to discriminate a type of a yarn defect of the yarn, based on a combination of thickness of the yarn and brightness of reflected light that has been reflected by the yarn when the yarn has been irradiated with light, characterized in that the discrimination section includes a first defect-discrimination-target identification section and a second defect-discrimination-target identification section, the first defect-discrimination-target identification section identifies, in a first variation signal for thickness, a first defect-discrimination-target portion for which a defect is to be assessed, the second defect-discrimination-target identification section identifies, in a second variation signal for brightness, a second defect-discrimination-target portion for which a defect is to be assessed, and the discrimination section identifies the first defect-discrimination-target portion and the second defect-discrimination-target portion that correspond to each other in a longitudinal direction of the yarn, and discriminates the type of the yarn defect based on the thickness of the first defect-discrimination-target portion and the brightness of the second defect-discrimination-target portion that have been identified as portions corresponding to each other, wherein the discrimination section determines that a first yarn defect having warpage, tangle, or entanglement protruding in a direction intersecting a direction in which the yarn travels is present when the thickness of the first defect-discrimination-target portion is equal to or greater than a first thickness criterion value and the brightness of the second defect-discrimination-target portion is equal to or higher than a first brightness criterion value. a discrimination section configured to discriminate a type of a yarn defect of the yarn, based on a combination of thickness of the yarn and brightness of reflected light that has been reflected by the yarn when the yarn has been irradiated with light, characterized in that the discrimination section includes a first defect-discrimination-target identification section and a second defect-discrimination-target identification section, the first defect-discrimination-target identification section identifies, in a first variation signal for thickness, a first defect-discrimination-target portion for which a defect is to be assessed, the second defect-discrimination-target identification section identifies, in a second variation signal for brightness, a second defect-discrimination-target portion for which a defect is to be assessed, and the discrimination section identifies the first defect-discrimination-target portion and the second defect-discrimination-target portion that correspond to each other in a longitudinal direction of the yarn, and discriminates the type of the yarn defect based on the thickness of the first defect-discrimination-target portion and the brightness of the second defect-discrimination-target portion that have been identified as portions corresponding to each other, wherein the discrimination section determines that a first yarn defect having warpage, tangle, or entanglement protruding in a direction intersecting a direction in which the yarn travels is present when the thickness of the first defect-discrimination-target portion is equal to or greater than a first thickness criterion value and the brightness of the second defect-discrimination-target portion is equal to or higher than a first brightness criterion value.