The stone chamber was made by fitting together 18 blocks of volcanic tuff.
Tuff from Mount Nijosan, on the border between Nara and Osaka, was used, and the chamber ceiling was worked in the form of a roof. The outer side of the southern ceiling stone was also carved in the shape of a roof. Gaps between the stones were filled with plaster.
The stone blocks were fit together with the “lap joint” technique.
In this method, still used today, the joining portions of the two members are cut away at right angles to form overlapping steps.
This method was used to fit the stones firmly together to make the chamber.