In digital holographic microscopy, the aperture synthesis technique improves resolution by extending the Fourier spectral area beyond the numerical aperture of the objective lens. We propose a method to locate the shifted spectra with subpixel accuracy at high speed. When spectra are synthesized at wrong locations, the synthesized image is distorted, for example, the intensity in the outer area is attenuated, limiting the field of view. We propose an accuracy criterion for the location of the spectra required to reduce this effect below a certain level. Then we calculated each spectral shift from the unscattered component with subpixel accuracy and resampled them so that their spacing became an integer in pixels. We demonstrated this method experimentally and achieved high-quality synthesized image with uniform intensity over the entire area. This method is fast compared to the others because the data used in the calculation are small and the process is simple.