The overall mass-transfer process of methane decomposition on Ni surfaces and hydrogen permeation through a Ni tube was experimentally investigated to design a catalytic-permeable Ni tube reactor. This is a basic study of an impurity detritiation system to decompose tritiated methane and continuously recover tritium in a gas mixture exhausted from fusion plasma. The mass-transfer process was comparatively studied under the two conditions of an open Ni tube without any packing and a Ni tube packed with 200-240 mesh Ni particles. Results were discussed in terms of a CH4 decomposition ratio decomp and a H2 permeation ratio perm. The decomp values depended on temperature and were almost independent of the flow rate. The decomp value was correlated to the first-order reaction-rate constant. On the contrary, the perm values were in reverse proportion to the flow rate and were almost independent of temperature. The perm value was related to diffusion through a H2 concentration boundary layer formed in the vicinity of the Ni tube wall. The degradation of catalytic performance due to carbon deposition on Ni was discussed based on our experiments.