In optics, the Fabry-Perot interferometer is a basic building block, enabling a selection of light with a specific wavelength. We theoretically propose a magnonic version of the Fabry-Perot interferometer based on two magnetic domain walls. By exploiting the interaction of a domain wall and spin waves, in particular, the phenomenon that a sufficiently narrow domain wall serves as an effective mirror for spin waves as it reflects spin wave almost completely, a magnonic interferometer is contrived by using two parallel domain walls. It allows tunability of the magnon wavelength passing through the interferometer by changing the distance between two domain walls. Our result offers a way to improve the ability of magnonic information processing in a simple magnetic structure.