  We have developed a formalism to study an inflationary scenario driven by a
bulk inflaton in the two-brane system. The 4-dimensional low energy effective
action is obtained using the gradient expansion method. It is also found that
the dark radiation and the dark scalar source are expressed by the radion. In
the single-brane limit, we find these dark components disappear. Therefore, it
turns out that the inflation due to the bulk inflaton successfully takes place.
Kaluza-Klein corrections are also taken into account in this case.
