PURPOSE: To form extremely narrow rubber width part at weaving part of rubber yarn of socks, etc., and to reduce cost in feeding rubber yarn, etc., to a knitting machine such as socks knitting machine by applying tension to rubber yarn in the direction opposite to that of rubber yarn feeding.

CONSTITUTION: In supplying elastic yarn (rubber yarn for short) Y such as rubber to a knitting machine such as socks knitting machine, tension is applied to the rubber yarn Y in the direction opposite to that of the rubber yarn Y feeding to the knitting machine and the rubber yarn is supplied. To be concrete, for example, in case of a rubber yarn feeder 6 wherein the rubber yarn is supplied to a knitting machine by rotation of a rubber yarn guide roll 2 with rotation of a rubber yarn feeding rotor 1, a drive motor 3 to revolve the rotor 1 consists of a step motor and tension is applied to the rubber yarn in the direction opposite to that of yarn feeding by reverse rotation of the motor 3.