An approximate in-resist image calculation method is proposed. From the configuration of light wave propagation in resist, sine of incident angle becomes 1/n times smaller than that in the air. Wavelength in resist is 1/n times shorter than that in the air; where n is refractive index of a resist. If wavelength in the air is 1/n times shorter at mask, sine of diffraction angle becomes 1/n times smaller, which is the same as the propagation angle of the diffracted light in resist for actual wavelength. Therefore, aerial image with 1/n times shorter wavelength under 1/n times smaller NA optics may become a good approximation of in-resist image for original wavelength, while attention should be required to image variation accompanied with z-shift of not observation point but z-shift of working wafer by which DOF is defined. We find that z-scale should be shrunk to obtain approximate DOF by a usual scaling law that is derived from phase relation of 3 beam interference with the largest interference at the optics. As an application of this calculation method, high NA ArF imaging, which will be realized by immersion optics, is investigated. Some interesting results, which may affect development strategy of ArF immersion lithography, are obtained in imaging characteristics. For example, resist blur in ArF resist, which seems to be much larger than that in KrF resist, may become the most serious problem to resolve a fine pattern with an extreme high NA ArF optics.