  Various spin gap excitations have been observed in the two-dimensional dimer
system SrCu_2(BO_3)_2 by means of submillimeter wave ESR. The zero-field energy
gap of the lowest spin gap excitation shows a splitting into two triplet modes
and the energy splitting clearly depends on the magnetic field orientation when
a field is rotated in the {\mib {ac}}-plane. A zero-field splitting is also
found between the S(_z)=+1 and S(_z)=-1 branches of each triplet. These
behaviors are qualitatively explained by considering the anisotropic exchange
coupling of inter-dimer and intra-dimer, respectively. The averaged value of
the lowest spin gap energy is determined to be 722 \pm 2 GHz(34.7 K). We have
also found the second spin gap excitation at 1140 GHz(54.7 K), which indicates
that the inter-dimer coupling is significantly strong. Besides these modes, a
number of gapped ESR absorption are found and we propose that these multiple
magnetic excitations are caused by the localized nature of the excited state in
the present system.
