  We have observed a "$K^-pp$"-like structure in the $d(\pi^+,K^+)$ reaction at
1.69 GeV/$c$. In this reaction $\Lambda(1405)$ hyperon resonance is expected to
be produced as a doorway to form the $K^-pp$ through the $\Lambda^*p\rightarrow
K^-pp$ process. However, most of the produced $\Lambda(1405)$'s would escape
from deuteron without secondary reactions. Therefore, coincidence of
high-momentum ($>$ 250~MeV/$c$) proton(s) in large emission angles
($39^\circ<\theta_{lab.}<122^\circ$) was requested to enhance the
signal-to-background ratio. A broad enhancement in the proton coincidence
spectra are observed around the missing-mass of 2.27 GeV/$c^2$, which
corresponds to the $K^-pp$ binding energy of 95 $^{+18}_{-17}$ (stat.)
$^{+30}_{-21}$ (syst.) MeV and the width of 162 $^{+87}_{-45}$ (stat.)
$^{+66}_{-78}$ (syst.) MeV.

Nuclear Experiment