PURPOSE: To prevent disturbance in orientation of fiber and change in width caused in producing a formed product of a modified cross section and obtain a readily formable unidirectional reinforcing base material having drapeability.

CONSTITUTION: Yarn groups in which both end yarns 2 of unidirectional reinforcing fiber 1 are covered with low-melting polymer yarn and yarn groups in which auxiliary yarn fiber 3 is covered with thermoplastic low-melting polymer yarn are heated to construct the subject base material of a woven fabric form in which the unidirectional reinforcing fiber 1 is bound to the auxiliary yarn fiber 3 and the above-mentioned both end yarns 2 are bound to the auxiliary yarn fiber 3 in respective interlaced parts.