The K̄NN bound system, symbolically denoted as "K-pp", is the simplest K̄-nuclear bound system which has been widely discussed as a consequence of the strongly attractive K̄N interaction in I = 0 channels. Many theoretical works have pointed out the existence of the "K-pp" bound system, but the calculated properties such as the binding energy and the width spread out due to the uncertainty of the K̄N interaction below the K̄+N mass threshold. Experimentally, there are several reports on observation of a "K-pp" candidate with the binding energy of around 100 MeV, however, no definitive evidence was available so far. At J-PARC, we conducted a experimental search for the "K-pp" bound system using K- + 3He reaction at 1.0 GeV/c where the "K-pp" is expected to be directly produced via the (K-, n) reaction. We finally observed a bound state below the K- +p+p mass threshold in the Λpn final state, which can be interpreted as the "K- pp" bound state. The possible existence of the "K- pp" state is discussed from both aspects of production and decay.