  When a hydrogen atom trapped in finite space formed by two parallel perfectly
conducting plates, the transition rates can be modified. Life-time of the
hydrogen $2P_{1/2}$-state is estimated to be shorter as much as $56.3ps $ with
a separation distance of $b = 1.2 \mu m$ between two plates. Although the
modification is dependent on position of the atom, the life-time can be shorten
or lengthen by adjusting the separation distance even for the case that the
atom is placed at the center between two plates.
