PURPOSE: To improve heat insulating property and flexibility at a high temperature by forming a sleeve having a desired shape by braiding long fibers chiefly comprising high purity alumina into a hollow string, and filling the inside of the hollow string with short fibers chiefly including predetermined purity alumina.

CONSTITUTION: High purity alumina long fibers containing 99.0wt% or more alumina are braided into a hollow core string. The long fibers have a length of the order of meter for example and tensile strength which is lowered with a heat treatment at 1500°C for 10 hours provided it is with lower alumina purity but is not lowered with high impurity alumina. The hollow core part of the core comprising the long fibers is filled with short fibers containing 90.0wt% or more alumina at a filling density of 0.20-0.30gr/cc to form a hollow string which string is thereafter formed into 3 desired sleeve. Hereby, a ceramic heat insulating sleeve is ensured which is improved in heat insulating property and flexibility at high temperature.