In about 1938, Yukawa Laboratory formed an idea that the particle (meson), which mediates nuclear forces, is a vector particle with a spin. Comprehensive research activities on the nuclear force, beta decay, magnetic moment and the like were conducted in groups, and led to the writing of the paper, “On the Interaction of Elementary Particles IV.” The folder contains not only Yukawa’s handwritten drafts but also summary notes written by his co-researchers, Sakata and Taketani.