a yarn is provided with a hollow fiber including a plurality of filaments and one or more longitudinal voids between the filaments ; and a heat retention fiber being a fiber for emitting infrared light , the heat retention fiber including a plurality of filaments each being capable of retaining heat , the filaments having nanoscale powder capable of emitting near and far infrared light uniformly formed therein . the hollow fiber and the heat retention fiber are twisted together to form a yarn having heat retention capability .