Recent experiments at KEK [1,2] show distinct narrow peaks in the reactions He(stopped K^-,N)NNNK, which correspond to strange tribaryons S^0(3115) and S^1(3140) respectively. These are theoretically predicted and claimed to be deeply bound kaonic nuclei by Akaiashi et al.,[3]. Moreover, in the FINUDA experiment [4], a kaon-bound state K^-pp has been claimed to be observed. Still, there remains experimentally and theoretically to confirm these events are really the kaonic nuclear systems. At this stage the theree-body K^-NN system is the simplest and the most significant one to be studied theoretically, since we can learn basic physics such as KN interactions, decay properties in transparency without any complications due to medium effect. I will discuss the structure of the ground state (J=0) and an excited state (J=1) of the three-body K^-NN and its decay process to (pi+Lambda+proton). references;

[1] T. Suzuki, et al., Phys. Lett. B 597 (2004) 263.

[2] M. Iwasaki, et al., nucl-ex/0310018, Phys. Lett. B, submitted for publication.

[3] Y. Akaishi, et al., Phys. Lett. B 613 (2005) 140 and the references therein.

[4] M. Agnello, et al., Phys. Rev. Lett. 94 (2005) 212303.