To supply a natural feathered fiber insulator, presenting a better insulation performance than the conventional insulator, emitting less hazardous material when disposed and incinerated, being mostly biodecomposed when buried. For the manufacturing, natural feathers are degreased and washed with alcohol 1, and after drying, are processed into fiber shape 2. 1 to 99 weight % of this feathered fiber obtained as component material and 1 to 99 weight % of PP/PET core sheath composite fiber as binder are compounded, and mixed uniformly 3. The mixture is laminated to obtain a required density 4, and heat molded by a hot air dryer 5. Feathered fiber and core sheath composite fiber's core fiber get caught complexly, allowing to increase the whole aperture rate considerably. As feathered fiber is a natural organic substance, it does not emit hazardous material when incinerated, and biodecomposed if buried in the earth.