Abstract
In this report we derive approximation of the transmission probability K for a cylindrical pipe which is usually given with computational calculation by using Monte Carlo technique nowadays, with rather old fashioned diffusion theory. Same results as Dushman's approximation is given based on the stationary solution of the one-dimensional diffusion equation with adopting the special boundary condition, that is the boundary of Dirichlet condition n=0 (n: number density of molecules) placing at the outer side of the real boundary (exit of the pipe). Transmission probability of the serially connected cylindrical ducts of different size is also given by those theory of diffusion.
