To provide an improved biodegradable heat insulator.

This improved biodegradable heat insulator contains 50 to 90 wt.% cellulose with an average fiber diameter of not more than 1 mm and an average fiber length of not more than 20 mm, at least, one kind of woody fiber, 2 to 15 wt.% flame retarder and 5 to 30 wt.% biodegradable binder of a two-component fiber-type with an average fiber diameter of not more than 1 mm and a fiber length of not more than 20 mm. In addition, the density of this heat insulator is 30 to 300 kg/m3.