  We discuss a Randall-Sundrum-type two D-braneworld model in which D-branes
possess different values of the tensions from those of the charges, and derive
an effective gravitational equation on the branes. As a consequence, the
Einstein-Maxwell theory is realized together with the non-zero cosmological
constant. Here an interesting point is that the effective gravitational
constant is proportional to the cosmological constant. If the distance between
two D-branes is appropriately tuned, the cosmological constant can have a
consistent value with the current observations. From this result we see that,
in our model, the presence of the cosmological constant is naturally explained
by the presence of the effective gravitational coupling of the Maxwell field on
the D-brane.
