The topology of cosmic reionization, i.e., whether high- or low-density regions of galaxies got ionized first (known as inside-out and outside-in scenarios), is one of the key remaining questions in early history of the universe. One can specify the scenario by the ionization state of the intergalactic medium (IGM) in galaxy overdensities (ODs). However, current observations can not provide a solid conclusion due to the lack of direct measurements in the IGM phase in galaxy ODs. We will address this question with a novel approach examining ODs of Lyα emitters (LAEs) associated with quasar metal absorption systems. The strength of this study is the capability to directly evaluate the IGM phase in galaxy ODs by Lyα opacity and various species of metal absorbers, allowing us to judge whether LAE ODs reside in ionized bubbles. In S23B, we propose to observe three quasar sightlines with metal absorbers at z=6.2 through the newly installed HSC/NB872 filter and examine our approach. We will verify the observational campaign in the following semesters by increasing the sample size and specify the reionization scenario based on the statistical sample.
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