1. A method of monitoring a focus position on a surface of a wafer for an exposure apparatus which transfers by exposure, through a projection optical system, a pattern formed on a mask onto the wafer, comprising: tilting, based on a measurement of a level detection sensor, an exposure area on the wafer and performing exposure while the mask and the exposure area have a relative angle, to form two spurious resolution images of the pattern of the mask in the exposure area; measuring, using a microscope, positions of the two spurious resolution images formed in the exposure area; and detecting an optimal focus position of the exposure apparatus on the basis of a middle point between the measured positions of the two spurious resolution images.