  We have made a survey of ceramics for the inner parts of the
transmission-type pressure cell to achieve the high pressure and the high
transmission in THz range. By using the optimal combination of ZrO$_{2}$-based
ceramic and Al$_{2}$O$_{3}$ ceramic, we have succeeded in obtaining the
pressure up to 1.5 GPa and the frequency region up to 700 GHz simultaneously.
We show the high-pressure ESR results of Shastry-Sutherland compound
SrCu$_{2}$(BO$_{3}$)$_{2}$ as an application. We observed the direct ESR
transition modes between the singlet ground state and the triplet excited
states up to the pressure of 1.51 GPa successfully, and obtained the precise
pressure dependence of the gap energy. The gap energy is directly proved to be
suppressed by the pressure. Moreover, we found that the system approaches the
quantum critical point with pressure by comparing the obtained data with the
theory. This result also shows the usefulness of high-pressure ESR measurement
in THz region to study quantum spin systems.
