The reduction of the magnetic ordering temperature has been studied in the quasi-two-dimensional square lattice Heisenberg magnet, Mn(HCOO)(2)(2)(NH2)(2)CO(T-N(1)=3.77 K) with the spin value S=5/2, randomly diluted with non-magnetic Cd2+, ions. It is found that, as the dilution proceeds, the Neel temperature TN(x) goes down toward 0 K around at x=0.65, higher than x(p)=0.59 for the square lattice, making the contrast to the cases of the diluted Ising system and La2CuO4, a high Tc superconducting parent compound with S=1/2: The critical index phi in TN(x)similar to(x-x(p))1/phi is suggested phi<1 for the Heisenberg and phi>1 for the Ising systems.