Japanese name: ウォーム！ウォーム！

80% acrylic / アクリル　80％
20% other / 指定外繊維（エクス）　20％

Note: the Japanese label says “20% other fiber “eks”, which is a name-brand Japanese synthetic fiber. The manufacturers claim that eks fiber absorbs moisture (such as sweat) and then uses that moisture to generate heat. According to this site, eks is a variant of polyester.