The crystal and electronic structures and electrical resistivity of new chlorine complex, (BEDT-TTF)3Cl2.5(H5O2), were investigated. The two-dimensional donor sheet and the anion layer stack alternately along the b-axis. The packing pattern of donors is similar to that of (BEDT-TTF)3(ClO4)2 and the sharp metal-insulator transition was observed at 170 K. It is interesting that not only donor and anion but also the third component cation, H+, determines the 3/4-filled band structure.