The construction of Tokyo Bay Fortress started in Meiji 13 (1880) from Kannonzaki battery and 24 batteries were con- structed. Although sea forts were important positions in that, when the range of the cannon became long, the important defense line was moved south from the bay mouth connecting Yokosuka to Futtsu to the line connecting the tip of the Miura Peninsula down to the southern part of the Boso Peninsula. They completed No.2 sea fort construction in Taisho 3 (1914), however there was no chance to exert defense capability in an actual combat situation.