FermiSurfer is a newly developed Fermi-surface viewer designed to facilitate the understanding of the physical properties of metals. It can display the Fermi surfaces of a material, color plots of arbitrary k-dependent quantities, the Fermi surface at arbitrary cross sections (Fermi lines), cross- or parallel- eye three-dimensional stereogram views, nodal lines, extremal orbits, and highlight the occupied or empty side of the Fermi surface. In addition, various first-principles software packages can produce input for FermiSurfer. This paper explains how to use FermiSurfer and demonstrates its usefulness by investigating the origin of the anisotropic superconductivity of YNi2B2C.