It is shown, by the theoretical consideration and by some numerical verifications, that when a well is incorporated in a network of a numerical model for plane groundwater flow by a simple term in the continuity equation for a node, this can be considered to mean that the well is supposed to have a certain radius, the value of which is a constant times of the mesh width. Especially for the network of square mesh, the equivalent radius is about 0.2 times of the mesh width.