places 2 putting the weighed fiber material into a glass beaker having a weight of 500 g 3 putting about 400 cc of distilled water at a temperature of 20 25 u00b0 c into the glass beaker prepared in the step 2 and dispersing the fiber material into the distilled water by an ultrasonic cleaner if necessary while stirring carefully by a stirrer so that the fiber material is not cut 4 transferring the content of the beaker prepared in the step 3 into a 1 000 ml graduated measuring cylinder and adding distilled water into the graduated measuring cylinder up to the scale of 1 000 cc 5 ten times repeating a process of stirring the content of the graduated measuring cylinder prepared in the step 4 by turning the graduated measuring cylinder upside down while blocking an opening of the graduated measuring cylinder with the palm of a hand or the like carefully to prevent water from leaking out 6 measuring the sedimentation volume of fiber by eye observation after placing the graduated measuring cylinder quietly under room temperature for 30 minutes after the stop of the stirring and 7 applying the aforementioned procedure to three samples and taking an average of the measured values as a measured value the mixture ratio of alumina fiber to mullite fiber is preferably selected to be in a range of from 10 90 to 90 10 in order to have heat resistance and surface pressure in good balance if the mixture ratio is out of the range the object of the invention cannot.