Tritium retention of carbon dust co-deposited with fuel hydrogen is large, and then it is required to evaluate the tritium inventory as a safety issue of ITER. Several species of co-deposited carbon dust were prepared by D2 arc discharge with carbon electrodes. The dependence of D2 gas pressure on the retained deuterium amount of the co-deposited dust was investigated. The structure and the surface morphology were also examined. The retained deuterium amount increased with the discharge gas pressure. The deuterium concentrations of the co-deposited carbon dust samples prepared at 1.3 Pa and 6.8 Pa were 0.12 and 0.3 in the atomic ratio, D/C, respectively. No clear dependence of the substrate temperature on retained deuterium amount was observed, perhaps due to the coarse temperature control. In the environment of gas pressure in ITER, approximately 1 Pa, the tritium concentration is estimated approximately T/C ≈ 0.06, which is several times smaller than the value estimated so far, T/C ≈ 0.2.