Nova Cas 2020 was a second example of a classical nova forming both C_2_ and CN molecules during its near-maximum phase. The formation C_2_ and CN is indicative of the carbon-rich (C/O>1) gas envelope of the nova. Our low-resolution spectroscopic observations in the optical from UT 2020 July 31 to August 19 revealed the appearance and the disappearance of the molecular absorption bands of C_2_ and CN in Nova Cas 2020. These molecules were present during ~3days only. Based on analysis of the C_2_ Swan band profiles, C_2_ (and probably CN also) formed at ~5000K in the nova envelope (see ~8000K for typical novae at their visual brightness maxima). Spectral evolution and the formation conditions of the molecules are similar to those of V2676 Oph, which is the first example of a C_2_/CN-forming nova. We predict that a late-phase grain formation episode similar to that seen in V2676 Oph will occur in Nova Cas 2020.