  Characteristics of the cascade gamma-ray signal resulting from
very-high-energy gamma-ray sources, such as gamma-ray bursts, can be used to
constrain the strength and structure of intergalactic magnetic fields (IGMF).
There has been a debate on whether GRB 190114C, the first gamma-ray burst with
observed TeV photons, can constrain the IGMF. Recently, LHAASO detected the
brightest-of-all-time GRB 221009A, which has much larger energy in TeV band and
the spectrum extends to energy above 10 TeV, providing an unprecedented
opportunity to studying IGMF. We perform a Monte-Carlo simulation of the
cascade process with the public ELMAG code, considering the TeV data of GRB
221009A observed by LHAASO. By comparing the resulting cascade emission with
the flux limit obtained from Fermi-LAT observations, we infer a limit of $B\ge
10^{-18.5}\rm G$ for IGMF. Though this limit may not be as strong as the limit
from blazars, it serves as an independent constraint on IGMF from a new class
of TeV sources.
