  When an electric field of light with a frequency of{\hbar}{\Omega}is applied
to a solid, Floquet states, consisting of sidebands with an interval of
{\hbar}{\Omega} around an electronic state, are expected to be formed. However,
only a few studies have experimentally detected such sidebands. Here, we apply
mid-infrared pump near-infrared reflection probe spectroscopy to a
one-dimensional Mott insulator,
bis(ethylendithio)tetrathianfulvalence-difluorotetracyanoquinodimethane
(ET-F2TCNQ), to detect the transient change in reflectivity R,{\Delta}R/R, due
to the formation of excitonic Floquet states. Analyses, considering both odd-
and even-parity excitons, demonstrate that the {\Delta}R/R spectrum reflects
the formation of the first-order Floquet sidebands of excitons, and its
spectral shape strongly depends on the widths of excitonic states. The
experimental and analytical approach reported here is effective in
demonstrating excitonic Floquet states in various solids.
