A position detection apparatus comprises an obtaining unit that obtains an image of a moiré that occurs due to first and second diffraction gratings overlapping; and a processor configured to perform a periodic analysis of a luminance distribution of the obtained image of the moiré, obtain a phase measurement value of the luminance distribution, and obtain a relative positioning between the first and second diffraction gratings, wherein the processor determines a phase shift amount for making the obtained phase measurement value be a value outside of a predetermined range including a discontinuous part of the phase, and performs a phase shift to shift the obtained phase measurement value by the determined phase shift amount, and obtains the relative positioning based on the phase-shifted phase measurement value.