The ρ Oph dark cloud is located at a distance of 160 pc and is known as a site of active formation of low-mass stars. In optical photographs a central core of a ∼ 1° × 1° extent and two thin filamentary elongations of obscuring matter are apparent. The two filaments extend toward the east over ∼ 4° – 10°. The central core has been studied in the 2.6 mm CO lines (e.g., Wilking and Lada, 1980, Ap. J. 224, 698). On the other hand, the two filaments have not yet been mapped in molecular spectra. Interestingly, optical studies revealed that polarization vectors run along these remarkable filaments, suggesting that the magnetic field plays an important role in determining the structure of the filaments.