In the J-PARC E15 experiment, a K- pp search was performed via the 3He(K-, n) reaction at 1.0 GeV/c. A forward-going neutron is detected by a neutron counter with 15 m flight length, and decay particles from K- pp are simultaneously measured by a cylindrical detector system that surrounds a liquid 3He target system. In March and May, 2013, we carried out the first physics data-taking with 5×109 incident kaons on the 3He target, and we have obtained a preliminary exclusive analysis result of 3He(K-, Λp)n reaction.