  Large High Altitude Air Shower Observatory has detected $0.2-13$ TeV emission
of GRB 221009A within 2000 s since the trigger. Here we report the detection of
a 400 GeV photon, without accompanying prominent low-energy emission, by Fermi
Large Area Telescope in this direction with a 0.4 days' delay. Given an
intergalactic magnetic field strength of about $4 \times 10^{-17}$ G, which is
comparable to limits from TeV blazars, the delayed 400 GeV photon can be
explained as the cascade emission of about 10 TeV gamma rays. We estimate the
probabilities of the cascade emission that can result in one detectable photon
beyond 100 GeV by Fermi Large Area Telescope within $0.3-1$ days is about 2$\%$
whereas it is about 20.5$\%$ within $0.3-250$ days. Our results show that
Synchrotron Self-Compton explanation is less favored with probabilities lower
by a factor of about $3-30$ than the cascade scenario.
