  The probability distribution functions of the circulation of velocity in
three-dimensional decaying isotropic turbulence are examined by the database of
the numerical simulation based on the pseudospectral method. It is shown that
the standard deviation increases nearly as $A^{2/3}$ where $A$ is the area of
the loop. The PDFs change from exponential to gaussian as the size becomes
large. The former agrees well with the prediction by Migdal (1993), although
the latter does not match. A modification is proposed to explain the gaussian
distribution with the $A^{2/3}$ dependence.
