We have investigated the superconducting critical temperature T c of solid solution Nb 5 Ir 3−x Pt x O with the Ti 5 Ga 4-type structure. The both end-members of Nb 5 Ir 3 O and Nb 5 Pt 3 O are reported to be superconductors with T c of 10.5 K and 3.8 K, respectively. Particularly Nb 5 Ir 3 O is considered as a two-gap superconductor. The entire series of alloy hold the Ti 5 Ga 4-type structure and show the linear x dependence of lattice parameters. On the other hand, a nonlinear T c vs x plot was obtained, suggesting that T c is not determined by only lattice parameters. The experimental result is discussed based on the Matthias rule and compared with those of the other two-gap superconductors such as MgB 2 , Mo 8 Ga 41 and Nb 3 Sn.