. A metal sheathed chromel-alumel (K) thermocouple (0 mm in outside diameter) was inserted into the catalyst layer to measure the catalyst layer temperature. Air was used as the oxygen-containing gas and a mixed gas so prepared as to have an oxygen molecule/carbon ratio of 0.00/0 was used as the reaction gas.  The reaction gas was preheated to 000° C. and supplied at a space velocity (STP) of 00