  The present paper deals with some kind of quantum ``velocity'' which is
introduced by the method of hydrodynamical analogy. It is found that this
``velocity'' is in general irrotational, namely, a vorticity vanishes, and then
a velocity potential must exist in quantum mechanics. In some elementary
examples of stable systems we will see what the ``velocities'' are. In
particular, the two-dimensional flows of these examples can be expressed by
complex velocity potentials whose real and imaginary parts are the velocity
potentials and stream functions, respectively.
