We have collected the first physics data of an experimental search for the simplest kaonic nuclear bound state, "K- pp", by the 3He(K- n) reaction at J-PARC. We confirmed that our spectrometer system works as designed and observed clear peak structure composed of the quasi-elastic K-"n" → K-n and the charge-exchange K-"p" → ̄K0n reactions in the forward neutron spectrum. © Owned by the authors, published by EDP Sciences, 2013.