To estimate the pore size of a mircofiltration membrane easily, a method based on a pure water permeation test has been developed. The pore size of a symmetric ceramic membrane was estimated with high accuracy by applying Darcy's law and the Kozeny-Carman equation to the measurements of pure water permeability. Based on this method, the pore size of a double layer membrane composed of two symmetric membranes was evaluated. The pore size of the top layer membrane was estimated by measuring the pure water permeation rates of the double layer membrane and the bottom layer membrane alone. Moreover, the pore size of a very thin skin layer coated on a supporting layer in an asymmetric membrane was estimated using a similar method.