Sterile neutrino is an example of decaying dark matter (DM) appears in a minor extension of the standard model. Although sterile neutrino DM has been probed by X-ray background, the simplest sterile neutrino DM poses a drawback in its cosmological production. New gauge interaction is a promising cure for it. If the new gauge boson is light enough, sterile neutrino DM decays into neutrinos efficiently. Then, in addition to sterile neutrino searches through photons, neutrinos originated from DM decay also can be detected in deep underground experiments such as DM direct detection experiments or the SK. Those multiple searches work complementarily.