Provided is a resin acid ester-based tackifier having a good compatibility with a synthetic rubber elastomer, being capable of reducing the melt viscosity of the elastomer, and being capable of imparting excellent adhesive properties andor pressure-sensitive adhesive properties to the elastomer. The cerium oxide-zirconium oxide-based mixed oxide comprises cerium oxide and zirconium oxide, wherein the weight ratio of CeO2: ZrO2 is 60:40 to 90:10, and the cerium oxide and the zirconium oxide are present as a mixture, the zirconium oxide being composed of a solid solution in which tetragonal or cubic zirconium oxide contains cerium. When a generated steam volume at a maximum detection time Tm+1 does not reach a standard steam volume A determined based on the maximum volume-maximum steam curve, it is judged that a large amount of food exceeding a maximum amount is being heated and the operation of the heating cooking apparatus is forcedly stopped. An optional peer-discovery algorithm is used to determine a content source for the requested content object within the CDN.