  Optomechanical systems are suitable for elucidating quantum phenomena at the
macroscopic scale in the sense of the mass scale. The systems should be
well-isolated from the environment to avoid classical noises, which conceal
quantum signals. Optical levitation is a promising way to isolate
optomechanical systems from the environment. To realize optical levitation, all
degrees of freedom need to be trapped. Until now, longitudinal trapping and
rotational trapping of a mirror with optical radiation pressure have been
studied in detail and validated with various experiments. However, less
attention has been paid to the transversal trapping of a mirror. Herein, we
report a pioneering result where we experimentally confirmed transversal
trapping of a mirror of a Fabry-P\'erot cavity using a torsional pendulum.
Through this demonstration, we experimentally proved that optical levitation is
realizable with only two Fabry-P\'erot cavities that are aligned vertically.
This work paves the way toward optical levitation and realizing a macroscopic
quantum system.
