TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The anionic functional group comprises sulfonate group, carboxyl group, sulfate group, thiol group, phosphate group, phosphonate group, iminodiacetic acid group, and aminophosphate group. (I) is made up of particles having an average particle size of 100-1000 mu m.    TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Method: The fuel flows through (I) from the downstream side to the upstream side of the metal removing filter in the vertical direction. Preferred Components: The container has mesh members, and a space between the meshes is filled with (I). The metal removing filter further comprises a fuel element which is placed at a downstream side of the fuel flow observed from the position of the metal removing agent in the container. The container: has a fuel inlet part (61) formed at a bottom surface of the container and a fuel outlet part formed at an upper surface of the container, and the fuel flows into the inside of the container through the fuel inlet part, then passes into the metal removing agent which is placed between the meshes, and passes into the fuel element, and finally flows out to the outside of the metal removing filter through the fuel outlet part; or comprises: a fuel inlet part formed at a central part of an upper surface of the container; fuel passage formed at the central part of the container along the longitudinal direction of the container so that the fuel passage communicates with the fuel inlet part, the metal removing agent which is placed between the meshes and the fuel element; and a fuel outlet part formed at the upper surface of the container, where the fuel flows into the inside of the container through the fuel inlet part and the fuel passage, passes into the metal removing agent which is placed between the meshes, then passes into the fuel element, and finally flows out to the outside of the metal removing filter through the fuel outlet part. EXAMPLE - None given.