  Searching for topological insulators/superconductors is a central subject in
recent condensed matter physics. As a theoretical aspect, various
classification methods of symmetry-protected topological phases have been
developed, where the topology of a gapped Hamiltonian is investigated from the
viewpoint of its onsite/crystal symmetry. On the other hand, topological
physics also appears in semimetals, whose gapless points can be characterized
by topological invariants. Stimulated by this background, we shed light on the
topology of nodal superconductors. In this paper, we review our modern
topological classification theory of superconducting gap nodes in terms of
symmetry. The classification method elucidates nontrivial gap structures
arising from nonsymmorphic symmetry or angular momentum, which cannot be
predicted by a conventional theory.
