(II) The problem of dejillillg the uniform rotation of the disk This problem has been investigated by generalizing relativistically some characteristic properties concerning the rotation of classical rigid body or incompressible fluid. In connection with this pro­ blem the law of velocity distribution of uniformly rotating disk is especially important. In the present stage, no decisive answer to these problems seems to exist. Since we are particularly interested in the problem of the velocity distribution and the contents of this paper are closely connected with it, we shall explain it somewhat in detail. Taking, now, Minkowski space-time M, its line element in a cylindrical coordinate system is written as