With respect to an oxide powder having a characteristic that a suspension suspending the oxide powder in pure water exhibits a pH value of 7 or less, a noble metal is loaded on the oxide powder by using a noble metal salt solution exhibiting a pH value lower than the pH value of the suspension in order to inhibit the granular growth of loaded noble metal particles at high temperatures. It is believed that the affinity enlarges between noble metal particles, generated by the decomposition of the noble metal salt, and the oxide powder because no coarse noble metal particles are generated by neutralizing the noble metal salt so that the binding force enlarges between the oxide powder and the noble metal salt.