  Since the report of the PeV-TeV neutrinos by the IceCube Collaboration,
various particle physics models have been proposed to explain the neutrino
spectrum by dark matter particles decaying into neutrinos and other Standard
Model particles.In such scenarios, simultaneous $\gamma$-ray emission is
commonly expected. Therefore, multi-messenger connections are generally
important for the indirect searches of dark matters. The recent development of
$\gamma$-ray astronomy puts stringent constraints on the properties of dark
matter, especially by observations with the Fermi $\gamma$-ray satellite in the
last several years. Motivated by the lack of $\gamma$-ray as well as the shape
of the neutrino spectrum observed by IceCube, we discuss a scenario in which
the DM is a PeV scale particle which couples strongly to other invisible
particles and its decay products do not contain a charged particle. As an
example to realize such possibilities, we consider a model of fermionic dark
matter that decays into a neutrino and many invisible fermions. The dark matter
decay is secluded in the sense that the emitted products are mostly neutrinos
and dark fermions. One remarkable feature of this model is the resulting
broadband neutrino spectra around the energy scale of the dark matter. We apply
this model to multi-PeV dark matter, and discuss possible observable
consequences in light of the IceCube data. In particular, this model could
account for the large flux at medium energies of $\sim10-100$ TeV, possibly as
well as the second peak at PeV, without violating the stringent $\gamma$-ray
constraints from Fermi and air-shower experiments such as CASA-MIA.
