For precise measurement of neutrino oscillation, understanding of neutrino interactions on nucleus is indispensable. One of the main systematic errors in neutrino oscillation analysis in the T2K experiment is induced due to the difference in the acceptance and target materials between Super-Kamiokande, with water target, and the near detector, with plastic target. In order to reduce this uncertainty, a new experiment, named WAGASCI, at the neutrino beamline in Japan Proton Accelerator Research Complex (J-PARC) has been proposed. Its goal is to measure the cross section ratio of charged current neutrino interaction on nucleus between water target and plastic target with a few percent uncertainty. The detector adopts three-dimensional grid-like configuration of scintillator around water and plastic targets, to obtain large angular acceptance. Multi-pixel photon counters (MPPC) will be used to detect the light from scintillator. In this paper, the design of the new detector optimized with Monte Carlo simulation, R&D of detector components, and the expected performance of the detector are reported.