It is important in relation to the plasma confinement to understand the spatial variation of the impedance observed by the ''stellarator diode method''. We develop a numerical code based on the collisional diffusion model and apply it to the various experimental conditions. It is shown that the ripple diffusion of electrons plays an important role in determination of the impedance between a magnetic surface and the vacuum vessel wall. The effects of the electron temperature, neutral pressure, helical ripple, and the magnetic island are also examined.