A cement for bonding an arc tube body made of an aluminum nitride sintered body and an electrode support made of molybdenum achieves high gas tightness in the obtainable arc tube without impairing the excellent translucency of the aluminum nitride sintered body. The cement contains a molybdenum powder and an aluminum nitride powder, and the total amount of metalloid elements, rare-earth elements and metal elements (except the rare-earth elements and aluminum element) corresponding to the following conditions (1) and (2) is 300 ppm or less: (1) metal elements having a melting point of 2000° C. or lower, and (2) metal elements having an ion radius smaller than that of aluminum.