  We investigate quantum phase transitions in the three-dimensional
orthogonal-dimer system of the compound $\rm SrCu_2(BO_3)_2$ by means of the
series expansion technique. We then discuss the phase diagram where the dimer
phase, the Haldane phase, the frustration-induced disordered phase and the
magnetically ordered phase compete with each other. It is found that the
compound $\rm SrCu_2(BO_3)_2$ is located in the dimer phase close to the phase
boundary.
