A filtration method for filtration of nucleated cells that includes providing a filter containing at least one of a metal and a metal oxide as a major component thereof and having a plurality of through-holes therein, and passing a liquid containing the nucleated cells through the filter. The diameter of an inscribed circle of each of the plurality of through-holes is smaller than the size of the nuclei of the nucleated cells, and the inscribed circle of each of the plurality of through-holes touches all sides defining an opening of the through-hole.