A yarn feeding device for an elastic yarn for a flat knitting machine, comprising: a sending-out mechanism that sends out the elastic yarn to a yarn feeding member by a roller based on knitting data that is used in a knitting machine body; a buffer arm that is swingable and that intermediately stores the yarn sent out from the sending-out mechanism; a torque generator that applies a torque to the buffer arm for causing a knitting tension of the elastic yarn from the sending-out mechanism to the yarn feeding member to become a predetermined value; and controlling means that calculates a theoretical value of a loop length of a stitch that is knitted for each knitting needle, and controls the sending-out mechanism such that a required yarn feeding operation of the elastic yarn is performed in synchronism with a movement of the yarn feeding member, characterized in that the yarn feeding device includes a tension sensor that is disposed on an upstream side of the roller, and that measures a tension of the elastic yarn as an entering tension of the elastic yarn to the roller, and correction amount determining means that compares a magnitude of the entering tension of the elastic yarn measured by the tension sensor and a magnitude of a knitting tension provided by the buffer arm, and, from a result of comparison, determines a correction amount with respect to the sending-out mechanism from a correction parameter, wherein the correction parameter is used by the correction determining means for correction for bringing the loop length of the stitch closer to the theoretical value from characteristics of the elastic yarn, wherein the correction parameter includes a correction parameter for increasing an amount of elastic yarn when the entering tension is larger than the knitting tension, and a correction parameter for decreasing the amount of elastic yarn when the entering tension is smaller than the knitting tension, wherein the correction amount determining means determines an appropriate correction amount from the correction parameter, and wherein the controlling means controls the sending-out mechanism by using the correction amount determined by the correction amount determining means. a sending-out mechanism that sends out the elastic yarn to a yarn feeding member by a roller based on knitting data that is used in a knitting machine body; a buffer arm that is swingable and that intermediately stores the yarn sent out from the sending-out mechanism; a torque generator that applies a torque to the buffer arm for causing a knitting tension of the elastic yarn from the sending-out mechanism to the yarn feeding member to become a predetermined value; and controlling means that calculates a theoretical value of a loop length of a stitch that is knitted for each knitting needle, and controls the sending-out mechanism such that a required yarn feeding operation of the elastic yarn is performed in synchronism with a movement of the yarn feeding member, characterized in that the yarn feeding device includes a tension sensor that is disposed on an upstream side of the roller, and that measures a tension of the elastic yarn as an entering tension of the elastic yarn to the roller, and correction amount determining means that compares a magnitude of the entering tension of the elastic yarn measured by the tension sensor and a magnitude of a knitting tension provided by the buffer arm, and, from a result of comparison, determines a correction amount with respect to the sending-out mechanism from a correction parameter, wherein the correction parameter is used by the correction determining means for correction for bringing the loop length of the stitch closer to the theoretical value from characteristics of the elastic yarn, wherein the correction parameter includes a correction parameter for increasing an amount of elastic yarn when the entering tension is larger than the knitting tension, and a correction parameter for decreasing the amount of elastic yarn when the entering tension is smaller than the knitting tension, wherein the correction amount determining means determines an appropriate correction amount from the correction parameter, and wherein the controlling means controls the sending-out mechanism by using the correction amount determined by the correction amount determining means.