  We report on the near-infrared polarimetric observations of RCW 120 with the
1.4 m IRSF telescope. The starlight polarization of the background stars
reveals for the first time the magnetic field of RCW 120. The global magnetic
field of RCW 120 is along the direction of $20^\circ$, parallel to the Galactic
plane. The field strength on the plane of the sky is $100\pm26\,\mu$G. The
magnetic field around the eastern shell shows evidence of compression by the
HII region. The external pressure (turbulent pressure + magnetic pressure) and
the gas density of the ambient cloud are minimum along the direction where RCW
120 breaks out, which explains the observed elongation of RCW 120. The
dynamical age of RCW 120, depending on the magnetic field strength, is
$\sim\,1.6\,\mathrm{Myr}$ for field strength of $100\,\mu$G, older than the
hydrodynamic estimates. In direction perpendicular to the magnetic field, the
density contrast of the western shell is greatly reduced by the strong magnetic
field. The strong magnetic field in general reduces the efficiency of triggered
star formation, in comparison with the hydrodynamic estimates. Triggered star
formation via the "collect and collapse" mechanism could occur in the direction
along the magnetic field. Core formation efficiency (CFE) is found to be higher
in the southern and eastern shells of RCW 120 than in the infrared dark cloud
receiving little influence from the HII region, suggesting increase in the CFE
related to triggering from ionization feedback.
