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The Veneziano amplitude is derived from fixing one degree of freedom of PSL(2,R) symmetry by the insertion of a mostly BRST exact operator. Evaluating the five-point function which consists of four open string tachyons and this gauge fixing operator, we find it equals the Veneziano amplitude up to a sign factor. The sign factor is interpreted as a signed intersection number. The result implies that the mostly BRST exact operator, which is originally used to provide two-point string amplitudes, correctly fixes the PSL(2,R) gauge symmetry for general amplitudes. We conjecture an expression for general n-point tree amplitudes with an insertion of this gauge fixing operator.