  High-pressure and high-field electron spin resonance (ESR) measurements have
been performed on a single crystal of the orthogonal-dimer spin system
SrCu$_{2}$(BO$_{3}$)$_{2}$. With frequencies below 1 THz, ESR signals
associated with transitions from the singlet ground state to the one-triplet
excited states and the two-triplet bound state were observed at pressures up to
2.1 GPa. We obtained directly the pressure dependence of the gap energies,
finding a clear first-order phase transition at $P_{c}=1.85\pm0.05$ GPa. By
comparing this pressure dependence with the calculated excitation energies
obtained from an exact diagonalization, we determined the precise pressure
dependence for inter- ($J'$) and intra-dimer ($J$) exchange interactions
considering the Dzyaloshinski-Moriya interaction. Thus this system undergoes a
first-order quantum phase transition from the dimer singlet phase to a
plaquette singlet phase above the ratio $(J'/J)_{c}=0.660\pm0.003$.
