We report on the results of angle-resolved photoemission experiments on a quasi-one-dimensional (1D) [Formula presented]-chain compound [Formula presented], which shows a gigantic nonlinear optical effect. A “band” having about 500 meV energy dispersion is found in the first half of the Brillouin zone, but disappears at [Formula presented]. These spectral features are well reproduced by the [Formula presented] chain model with a small charge-transfer energy [Formula presented] compared with that of 1D Cu-O compounds. We propose that this smaller [Formula presented] is the origin of the absence of clear spin-charge separation in the photoemission spectra and the strong nonlinear optical effect. © 2002 The American Physical Society.