H-dibaryon has been predicted as a stable 6-quark color-singlet state. It has been searched for by many experiments but has never been discovered. Recent lattice QCD calculations predict H-dibaryon as a weakly bound or a resonant state close to the LL threshold. E224 and E522 experiments at KEK observed peaks in LL invariant mass spectra near the threshold in (K-, K+) reactions, which were statistically not significant. Therefore, we proposed a new experiment E42 at J-PARC. It will measure decay products of ΛΛ and Λπ-p in a (K-, K+) reaction. We design a large acceptance spectrometer based on a Time Projection Chamber (TPC) immersed in a dipole magnetic field. The TPC surrounds a target to cover nearly 4π acceptance, and accepts K- beams up to 106 counts per second. To suppress drift field distortion at high beam rates, we adopt Gas Electron Multipliers (GEMs) for electron amplification and a gating grid. We show an overview of the experiment, the design of the spectrometer, and the R&D status of the TPC prototype.