  The two dimensional Heisenberg model for SrCu_2(BO_3)_2 has the exact dimer
ground state which was proven by Shastry and Sutherland almost twenty years
ago. The critical value of the quantum phase transition from the dimer state to
the N\'{e}el ordered state is determined. Analysis of the experimental data
shows that SrCu_2(BO_3)_2 has the dimer ground state but is close to the
transition point, which leads to the unusual temperature dependence of the
susceptibility. Almost localized nature of the triplet excitations explains the
plateaus observed in the magnetization curve.
