A yarn supplying system having: a yarn supplying device including a motor for rotating a bobbin, around which a yarn is wound, to reel out a yarn from the bobbin, and a yarn storage device for temporarily storing the yarn reeled out from the bobbin and then feeding the yarn to a receiving device in a middle of a yarn supplying path, which extends from the yarn supplying device to the receiving device of the yarn, to moderate fluctuation in tension that acts on the yarn resulting from a difference between a supply amount of the yarn from the yarn supplying device and a usage amount of the yarn of the receiving device characterized in that the yarn storage device includes a storage unit for storing the yarn, a pulling piece, which is hooked to the yarn to pull the yarn into the storage unit and which linearly reciprocates along a pulling direction, and an elastic body for pulling the pulling piece in the pulling direction, and the yarn supplying system further includes tension measuring means for measuring a tension of the elastic body, deriving means for deriving a physical amount from a measurement result of the tension measuring means using a correlative relationship between the tension of the elastic body and the physical amount for controlling a supply amount of the yarn from the bobbin determined in advance, and control means for controlling the supply amount of the yarn from the bobbin based on the derived physical amount, wherein the position of the pulling piece along the pulling direction is adopted for the physical amount for controlling the supply amount of the yarn from the bobbin, and wherein the control means is adapted to control the motor based on the derived physical amount. a yarn supplying device including a motor for rotating a bobbin, around which a yarn is wound, to reel out a yarn from the bobbin, and a yarn storage device for temporarily storing the yarn reeled out from the bobbin and then feeding the yarn to a receiving device in a middle of a yarn supplying path, which extends from the yarn supplying device to the receiving device of the yarn, to moderate fluctuation in tension that acts on the yarn resulting from a difference between a supply amount of the yarn from the yarn supplying device and a usage amount of the yarn of the receiving device characterized in that the yarn storage device includes a storage unit for storing the yarn, a pulling piece, which is hooked to the yarn to pull the yarn into the storage unit and which linearly reciprocates along a pulling direction, and an elastic body for pulling the pulling piece in the pulling direction, and a storage unit for storing the yarn, a pulling piece, which is hooked to the yarn to pull the yarn into the storage unit and which linearly reciprocates along a pulling direction, and an elastic body for pulling the pulling piece in the pulling direction, and the yarn supplying system further includes tension measuring means for measuring a tension of the elastic body, deriving means for deriving a physical amount from a measurement result of the tension measuring means using a correlative relationship between the tension of the elastic body and the physical amount for controlling a supply amount of the yarn from the bobbin determined in advance, and control means for controlling the supply amount of the yarn from the bobbin based on the derived physical amount, wherein the position of the pulling piece along the pulling direction is adopted for the physical amount for controlling the supply amount of the yarn from the bobbin, and wherein the control means is adapted to control the motor based on the derived physical amount. tension measuring means for measuring a tension of the elastic body, deriving means for deriving a physical amount from a measurement result of the tension measuring means using a correlative relationship between the tension of the elastic body and the physical amount for controlling a supply amount of the yarn from the bobbin determined in advance, and control means for controlling the supply amount of the yarn from the bobbin based on the derived physical amount, wherein the position of the pulling piece along the pulling direction is adopted for the physical amount for controlling the supply amount of the yarn from the bobbin, and wherein the control means is adapted to control the motor based on the derived physical amount.