The open pore generation on the surface of Ti(C, N) base cermets by sintering was investigated as a function of the compositional and processing factors, and then its mechanism was considered. The following were obtained. (1) Phenomena: The generation occurred, when the atmosphere in the temperature-keeping stage during sintering was vacuum (7 Pa), irrespective of the kind of atmospheres (vacuum and nitrogen of 0.13 kPa) in the temperature-raising and cooling stages, and vice versa. The generation became more frequent at higher N/(C+N) atomic ratio of Ti(C, N) powder and/or at higher carbon content of cermet as far as the microstructure was without free carbon phase and (Ti, Mo)₃Ni compound, i.e., θ-phase. The origin of open pore was a dent on the surface of the green compact. (2) Mechanism: The growth of open pores occurs by the net-diffusion, i.e., transport of Ti and Mo atoms mainly from the side and bottom of the dent to the edge. This diffusion is caused by the N content gradient formed due to the denitriding, which occurs more rapidly from the compact surface having lager ratio of surface-area/volume. The method for suppressing the open pore generation was also considered.