  Recently anomalous flux in the cosmic optical background (COB) has been
reported by the New Horizon observations. The COB flux is $16.37\pm 1.47\, \rm
nW m^{-2} sr^{-1}$, at the LORRI pivot wavelength of $0.608\,\rm \mu m$, which
is $\sim 4\sigma$ level above the expected flux from the Hubble Space Telescope
(HST) galaxy count. It would be great if this were a hint for the eV scale dark
matter decaying into photons. In this paper, we point out that such a decaying
dark matter model predicts a substantial amount of anisotropy in the COB flux,
which is accurately measured by the HST. The data of the HST excludes the decay
rate of the dominant cold dark matter larger than $10^{-24}$-$10^{-23}\,{\rm
s}^{-1}$ in the mass range of $5$-$20$eV. As a result, the decaying cold dark
matter explaining the COB excess is strongly disfavored by the anisotropy
bound. We discuss some loopholes: e.g. warm/hot dark matter or two-step decay
of the dark matter, to explain the COB excess.
