A large volume radiation detector using a semi-insulating Indium Phosphide (InP) wafer has been developed for Indium Project on Neutrino Observation for Solar interior (IPNOS) experiment. We have achieved the volume of 20 mm(3), and this is world largest one among InP detectors which observed gamma's at hundred key region. In spite of the depletion layer, most of charge generated by electron hole pair production are collected by an induction, and the charge collection efficiency and the energy resolution are obtained by 60% and 25%, respectively. We measured actual backgrounds related to (115)In beta decay, and no significant background was found. (C) 2010 Elsevier B.V. All rights reserved.