A yarn winder which includes a winding unit configured to form a package by winding a yarn supplied from a yarn supplying unit onto a bobbin, and is capable of automatically replacing the completed package with a new bobbin, the yarn winder comprising: a bobbin holder which supports the bobbin to be rotatable; and a clamp cutter which is configured to cut a part of the yarn between the yarn supplying unit and the package and to hold a part of the yarn on the yarn supplying unit side, and then to guide the held yarn to either the bobbin holder or a yarn holding portion of the bobbin, when the package is replaced with the new bobbin; characterized by further comprising: a yarn slack adjustment unit which is configured to perform tensioning that is to remove slack of the yarn held by the clamp cutter and slackening that is to slack the yarn held by the clamp cutter; and a controller which is configured to control at least the yarn slack adjustment unit, wherein, the clamp cutter is arranged to perform at least: after the part of the yarn between the yarn supplying unit and the package is cut and held, first movement of moving in a direction which includes a movement component toward one side in an orthogonal direction orthogonal to an axial direction of the bobbin holder and is a direction of pulling the yarn out from the yarn supplying unit; and after the first movement, second movement of moving in a direction including a movement component toward the other side in the orthogonal direction in order to guide the held yarn to the yarn holding portion, and after the clamp cutter starts to perform the second movement and before the yarn holding portion holds the yarn, the controller causes the yarn slack adjustment unit to perform the tensioning. a bobbin holder which supports the bobbin to be rotatable; and a clamp cutter which is configured to cut a part of the yarn between the yarn supplying unit and the package and to hold a part of the yarn on the yarn supplying unit side, and then to guide the held yarn to either the bobbin holder or a yarn holding portion of the bobbin, when the package is replaced with the new bobbin; characterized by further comprising: a yarn slack adjustment unit which is configured to perform tensioning that is to remove slack of the yarn held by the clamp cutter and slackening that is to slack the yarn held by the clamp cutter; and a controller which is configured to control at least the yarn slack adjustment unit, wherein, the clamp cutter is arranged to perform at least: after the part of the yarn between the yarn supplying unit and the package is cut and held, first movement of moving in a direction which includes a movement component toward one side in an orthogonal direction orthogonal to an axial direction of the bobbin holder and is a direction of pulling the yarn out from the yarn supplying unit; and after the first movement, second movement of moving in a direction including a movement component toward the other side in the orthogonal direction in order to guide the held yarn to the yarn holding portion, and after the clamp cutter starts to perform the second movement and before the yarn holding portion holds the yarn, the controller causes the yarn slack adjustment unit to perform the tensioning. a yarn slack adjustment unit which is configured to perform tensioning that is to remove slack of the yarn held by the clamp cutter and slackening that is to slack the yarn held by the clamp cutter; and a controller which is configured to control at least the yarn slack adjustment unit, wherein, the clamp cutter is arranged to perform at least: after the part of the yarn between the yarn supplying unit and the package is cut and held, first movement of moving in a direction which includes a movement component toward one side in an orthogonal direction orthogonal to an axial direction of the bobbin holder and is a direction of pulling the yarn out from the yarn supplying unit; and after the first movement, second movement of moving in a direction including a movement component toward the other side in the orthogonal direction in order to guide the held yarn to the yarn holding portion, and after the clamp cutter starts to perform the second movement and before the yarn holding portion holds the yarn, the controller causes the yarn slack adjustment unit to perform the tensioning. after the part of the yarn between the yarn supplying unit and the package is cut and held, first movement of moving in a direction which includes a movement component toward one side in an orthogonal direction orthogonal to an axial direction of the bobbin holder and is a direction of pulling the yarn out from the yarn supplying unit; and after the first movement, second movement of moving in a direction including a movement component toward the other side in the orthogonal direction in order to guide the held yarn to the yarn holding portion, and after the clamp cutter starts to perform the second movement and before the yarn holding portion holds the yarn, the controller causes the yarn slack adjustment unit to perform the tensioning.