  We observed a distinct peak in the $\Lambda p$ invariant mass spectrum of
$^{3}{\rm He}(K^-, \, \Lambda p)n$, well below the mass threshold of $m_K + 2
m_p$. By selecting a relatively large momentum-transfer region $q = 350 \sim
650$ MeV/$c$, one can clearly separate the peak from the quasi-free process,
$\overline{K}N \rightarrow \overline{K}N$ followed by the non-resonant
absorption by the two spectator-nucleons $\overline{K}NN \rightarrow \Lambda N
$. We found that the simplest fit to the observed peak gives us a Breit-Wigner
pole position at $B_{\rm {\it Kpp}} = 47 \pm 3 \, (stat.) \,^{+3}_{-6}
\,(sys.)$ MeV having a width $\Gamma_{\rm {\it Kpp}} = 115 \pm 7 \, (stat.)
\,^{+10}_{-9} \,(sys.)$ MeV, and the $S$-wave Gaussian reaction form-factor
parameter $Q_{\rm {\it Kpp}} = 381 \pm 14 \, (stat.)\,^{+57}_{-0} \,(sys.)$
MeV/$c$, as a new form of the nuclear bound system with strangeness --
"$K^-pp$".
