  The first prototype of LST (LST-1) for the Cherenkov Telescope Array has been
in commissioning phase since 2018 and already started scientific observations
with the low energy threshold around a few tens of GeV. In 2021, LST-1 observed
BL Lac following the alerts based on multi-wavelength observations and detected
prominent gamma-ray flares. In addition to the daily flux variability, LST-1
also detected sub-hour-scale intra-night variability reaching 3-4 times higher
than the gamma-ray flux from the Crab Nebula above 100 GeV. In this proceeding,
we will report the analysis results of LST-1 observations of BL Lac in 2021,
especially focusing on flux variability.
