We report a first clear detection of the Zeeman splitting of a CCS emission line at 45 GHz toward a nearby prestellar dense filament , Taurus Molecular Cloud-1 .
We observed HC _ { 3 } N non-Zeeman line simultaneously as the CCS line , and did not detect any significant splitting of HC _ { 3 } N line .
Thus , we conclude that our detection of the CCS Zeeman splitting is robust .
The derived \textcolor blackline-of-sight magnetic field strength is about 117 \pm 21 \mu G , which corresponds to the normalized mass-to-magnetic flux ratio of 2.2 if we adopt the inclination angle of 45 ^ { \circ } .
Thus , we conclude that the TMC-1 filament is magnetically supercritical .
Recent radiative transfer calculations of CCS and HC _ { 3 } N lines along the line of sight suggest that the filament is collapsing with a speed of \sim 0.6 km s ^ { -1 } , which is comparable to three times the isothermal sound speed .
This infall velocity appears to be consistent with the evolution of a gravitationally-infalling core .