  It is assumed that superheavy dark matter particles with O(EeV) mass may
decay to relativistic milli-charged particles (MCPs). The upward-going MCPs
passing through the Earth could be measured by the fluorescence detectors (FD)
of the Pierre Auger observatory (Auger). The massless hidden photon model was
taken for MCPs to interact with nuclei, so that the numbers and fluxes of
expected MCPs and neutrinos may be evaluated at the FD of Auger. Based on the
assumption that no events are observed at the FD of Auger in 14 years, the
corresponding upper limits on MCP fluxes were calculated at 90\% C. L.. These
results indicated that MCPs could be directly detected in the secondaries'
energy range O(1EeV)-O(10EeV) at the FD of Auger, when
$3\times10^{-13}\lesssim\epsilon^2\lesssim 10^{-3}$. And a new region of 10 MeV
< $m_{MCP}$ < 1 PeV and $10^{-7}$ $\lesssim$ $\epsilon$ $\lesssim$
$3\times10^{-2}$ is ruled out in the $m_{MCP}$-$\epsilon$ plane with 14 years
of Auger data.
