Forming rate of inner tubes via the heating Of C-60 encased single-wall carbon nanotubes (peapods) can be controlled by the pre-heating in hydrogen gas. Once the peapods were heated in the hydrogen at 1100 degrees C, polymerization Of C-60 molecules drastically suppressed and this effect remains for a while even after exhausting the hydrogen gas. Then, the polymerization rate gradually recovered. This phenomenon reminds that the hydrogen gas effectively controls the chemical reaction in such a confined nano-space without destructing the original features.