  We present the effective equations to describe the four-dimensional gravity
of a brane world, assuming that a five-dimensional bulk spacetime satisfies the
Einstein equations and gravity is confined on the $Z_2$ symmetric brane.
Applying this formalism, we study the induced-gravity brane model first
proposed by Dvali, Gabadadze and Porrati. In a generalization of their model,
we show that an effective cosmological constant on the brane can be extremely
reduced in contrast to the case of the Randall-Sundrum model even if a bulk
cosmological constant and a brane tension are not fine-tuned.
