In this work, we present a numerical simulation of scanning near-field optical microscopy. The simulation is based on a recently developed macroscopic theory which rigorously solves the electromagnetic field in a two-dimensional dielectric-air-dielectric sandwich system with arbitrary one-dimensional structures at its two interfaces. A tiny dielectric triangular is added on one interface to represent the probe tip in the device, while on the other interface, two identical topographic defects with various profiles are attached to represent the sample surface. The probe tip is assumed scanning at a constant height above the sample surface which is illuminated by a p-polarized light. The intensity of the local field averaged along a cross section inside the tip is calculated to represent the signal level of the microscope. The feasibility of the theoretical scheme is numerically demonstrated, and various aspects in the microscopy, such as the effects of the separation of the two objects, and of the shape differences between ridges and grooves are discussed.