Catalysis on Ru clusters supported on CeO2 or Ni-doped CeO2 was investigated. RU(3)(CO)(12) was reacted with CeO2, followed by heating in vacuum at 673 (i) or 813 K (ii) and then in H-2 at 588-1073 K (T-H2). Activities of both catalysts had the T-H2 dependence, which has a maximum at T-H2 = 873 K for ammonia synthesis. The rates on i were faster than those on ii by a factor of 2.0-1.1 in the range of T-H2 = 588-973 K. On a sample in which Ru-3(CO)(12) was supported on previously reduced Ni/CeO2 (in H-2 at 773 K) iii, the highest synthesis rate was 1.5 x 10(-3) mol h(-1) g(cat)(-1) at T-H2 = 588 K on iii. The activity order iii > i > ii can be understood in terms of two factors : (A) reduction extent of support and (B) number of active Ru sites. The two factors conflicted with each other when the treatment temperature in H-2 increased. By heating the samples in H-2 up to 873 K to satisfy factor A, the aggregation of Ru clusters for i or physical blocking of surface Ru sites by CeO2-x for ii occurred : factor B was not satisfied. The two factors should be optimized in catalyst iii, where the support cerium oxide was thoroughly reduced through the doped Ni. On reduced Ni/CeO2, the Ru cluster implantation can be done at low temperature (588 K). Obtained values of r(Ru-Ru) at 2.62 Angstrom (N = 7.1) and r(RU)-(O(S)) (O(s) is the oxygen atom at surface) at 2.12 Angstrom (N = 1.2) by EXAFS for Ru-3-Ni/CeO2 suggested a flat Ru cluster model comprised of several Ru atoms on reduced Ni/CeO2-x surface. The H(a)/Ru-total ratio exceeded unity for catalysts i and iii, suggesting new H adsorption sites. The temperature-programmed desorption for hydrogen (simultaneous desorption of HD and D-2 for iii at 330-430 K suggested that the H at the new site and H on Ru surface were exchangeable above 330 K. The "reservoir" effect of the new site for H on catalysis is discussed in relation to new kinetic design of hydrogenation catalyst.