We have measured proton and neutron energy spectra by means of time-of-flight (TOF) from 4He( K stopped − , p / n ) reactions (KEK PS E471 experiment). In the proton spectrum, a clear mono-energetic peak was observed under semi-inclusive condition, which was assigned to the formation of a strange tribaryon S 0(3115) with isospin T = 1 . The mass and width of the state were deduced to be 3117.7 −2.0 +3.8 ( syst . ) ± 0.9 ( stat . )   MeV / c 2 and < 21.6   MeV / c 2 , respectively, and its main decay mode was Σ N N . In the neutron spectrum, a mono-energetic peak was found as the result of a detailed analysis, which was assigned to the formation of another kind of strange tribaryon S +(3140). The mass and width of the state were deduced to be 3140.5 −0.8 +3.0 ( syst . ) ± 2.3 ( stat . )   MeV / c 2 and < 21.6   MeV / c 2 , respectively, and its main decay mode was Σ ± N N . The isospin of the state is assigned to be 0. The results are compared with recent theoretical calculations.