High-field electron spin resonance (ESR) measurements of single crystals of (CH 3 ) 2 NH 2 CuCl 3 (DMACuCl 3 ) have been performed in the frequency range of 50 to 550 GHz in the paramagnetic region. In addition to the electron paramagnetic resonance, we have observed four ESR modes ω 1 , ω 2 , ω 3 , and ω 4 . The modes ω 2 and ω 3 have a frequency gap of 325 GHz (15.6 K), while the mode ω 1 has a gap of 420 GHz (20 K) at 0 T. The modes ω 2 and ω 3 are attributed to the transitions between the singlet and the triplet of the antiferromagnetic (AF) dimer. We found that the large spin-gap for the AF dimer still remains at the critical field B c2 = 3.5 T determined by magnetization and specific heat measurements.