The aqueous solubilities of CH4 in the two-phase (hydrate (H)-liquid water (LW)) region, which is very close to the deep sea floor condition, were measured at various ranges of temperature and pressure. The hydration number determined via Raman spectroscopy at 10.0 MPa and 274.15 K was found to be 6.00 that is a little higher than 5.75 of the ideal one. The solubility of CH4 in liquid water largely increased with a small increase of temperature, but slightly decreased with increasing pressure in the two-phase (H-LW) region. This solubility behavior was experimentally confirmed to be completely different from that occurring in the three-phase (H-LW-V) boundary. The present results might be valuable as the fundamental data for estimating the amount of in situ gas hydrate and understanding the unique feature of hydrate formation/ dissociation mechanism and the hydrate stability in the deep ocean sediments.