アブストラクト：
We developed a new scheme to evaluate z = 3.1 HI Lya absorption strength relative to the cosmic mean from photometric data of star-forming galaxies at 3.1 < z < 3.5. With the new measure, ΔNB497, we have made two-dimensional HI absorption maps in the z = 3.1 SSA22 porto-cluster region and two control fields with a spatial resolution of ~ 5 comoving Mpc. The HI absorption strength in the SSA22 field are systematically larger than those in the control fields, and this HI absorption enhancement extends more than 50 comoving Mpc. The field-averaged (i.e., ∼ 50 comoving Mpc scale) HI absorption strength and the overdensity of LAEs seem to be correlated, while there is no clear dependency of the HI absorption strength on the local LAE overdensity in a few comoving Mpc scale. These results suggest that diffuse HI gas spreads out in/around the SSA22 proto-cluster. We have also found an enhancement of HI absorption at a projected distance < 100 physical kpc from the nearest z = 3.1 galaxies, which is probably due to HI gas associated with the CGM of individual galaxies.