High-field and high-pressure ESR measurements of SrCu2(BO3)2

High-field and high-pressure ESR measurements of SrCu2(BO3)2 have been performed at 1.6 K in the field region up to 30 T, the frequency region from 70 to 360 GHz and the pressure region up to 12 kbar. The direct ESR transition mode between the singlet ground state and the triplet excited states was observed at various pressures. We have succeeded in observing the decrease of the spin gap directly upon applying the pressure. The origin of the decrease of the gap energy is discussed.