Transformation properties of the four-point Veneziano formula as well as its Koba-Nielsen variables are discussed in the language of modular-group theory. The modular group is here associated with Jacobi's theta functions which naturally appear in this area as a consequence of the functional-integral consideration on a rectangular world sheet. Invariance of Veneziano's differential form under the action of the modular group determines the value of the Regge intercept at which the Virasoro condition (also invariant) holds.