We prepared niobium carbide clusters, NbnCm, in the gas phase by a double laser ablation technique. A photoionization efficiency was measured as a function of the wavelength of an ionization laser to determine ionization energies (E-i's) of NbnCm (n = 3 - 10, m = 0-7). The E-i's of Nb4C4 and Nb5C3 are found to be the lowest in the clusters studied. When the experimental Ei's are compared with the E-i's estimated by the density functional calculation previously reported (Harris, H.; Dance, I. J. Phys. Chem. A 2001, 105, 3340-3358), we determined Nb5C3 to have a similar 2 x 2 x 2 cubic structure as Nb4C4. We also found that the E-i's of carbon-rich clusters, NbnCm (n <= m), tend to be higher (>5 eV) than the E-i's of the niobium-rich clusters. The high E-i's are due to the structure of the carbon-rich clusters: carbon-carbon bonding is preferred when the number of carbon atoms exceeds the number of metal atoms. The structure was also discussed in relation to the reactivity of cluster ions with a hydrogen molecule.