  A theoretical model is presented to explain the spin gap observed for
CaV$_4$O$_9$. The underlying lattice of the 1/5-depleted square lattice favors
a formation of plaquette resonating valence bond state. Inclusion of the
frustrating second neighbor interaction enhances this tendency, leading to a
quantum disordered state of a two dimensional spin-1/2 Heisenberg model with a
sufficiently big spin gap compatible with experiments.
