  The combination of a thin disk and a narrowly-collimated jet is a typical
structure that is observed in the vicinity of a massive object such as AGN,
black hole or YSO. Despite a large variety of their scales and possible
diversity of involved processes, a simple and universal principle dictates the
geometric similarity of the structure; we show that the singularity at the
origin ($r=0$) of the Keplerian rotation ($V_\theta \propto r^{-1/2}$) is the
determinant of the structure. The collimation of jet is the consequence of the
alignment ---so-called Beltrami condition--- of the flow velocity and the
"generalized vorticity" that appears as an axle penetrating the disk (the
vorticity is generalized to combine with magnetic field as well as to subtract
the friction force causing the accretion). Typical distributions of the density
and flow velocity are delineated by a similarity solution of the simplified
version of the model.
