A process for manufacturing a preform comprising a first reinforcing fiber base material having, in its cross-section, a web portion and at least one pair of flange portions (F1a, F1 b) extending in opposite side each other through at least one branching portion from the web portion, and at least one second reinforcing fiber base material that is united with the first reinforcing fiber base material in a state including the at least one branching portion between them wherein: (a) the first reinforcing fiber base material comprises a first raw base material and a second raw base material, (b) the first raw base material comprises a first layered strip comprising layered multiple reinforcing fiber cloths and an adhesive resin provided between the layered reinforcing fiber cloths, (c) the second raw base material comprises a second layered strip comprising layered multiple reinforcing fiber cloths and an adhesive resin provided between the layered reinforcing fiber cloths, (d) the second reinforcing fiber base material comprises at least one third raw base material, (e) a first supply step for supplying the first layered strip, a second supply step for supplying the second layered strip, and at least one third supply step for supplying the at least one third raw base material are provided, (f) a first partial shaping step is provided on the downstream side of the first supply step for shaping the first layered strip by heating and/or pressing and maintaining the shaped state, thereby producing a first preliminary shaped body, (g) a second partial shaping step is provided on the downstream side of the second supply step for shaping the second layered strip by heating and/or pressing and maintaining the shaped state, thereby producing a second preliminary shaped body, (h) a uniting step is provided on the downstream side of the first and second partial shaping steps and the at least one third supply step to unite the first and second preliminary shaped bodies (Pf1, Pf2) produced in the first and second partial shaping steps in such a way that the branching portion is formed in the preform and the at least one third raw base material is added at the branching portion formed, followed by uniting them by heating and/or pressing them in the configuration and maintaining the united state, thereby providing the preform, (i) a conveyance step is provided on the downstream side of the uniting step to intermittently convey the preform produced in the uniting step, and (j) the production of the first and second preliminary shaped bodies (Pf1, Pf2) in the first and second partial shaping steps and the production of the preform in the uniting step are carried out during the periods when the conveyance of the preform in the conveyance step is suspended. (a) the first reinforcing fiber base material comprises a first raw base material and a second raw base material, (b) the first raw base material comprises a first layered strip comprising layered multiple reinforcing fiber cloths and an adhesive resin provided between the layered reinforcing fiber cloths, (c) the second raw base material comprises a second layered strip comprising layered multiple reinforcing fiber cloths and an adhesive resin provided between the layered reinforcing fiber cloths, (d) the second reinforcing fiber base material comprises at least one third raw base material, (e) a first supply step for supplying the first layered strip, a second supply step for supplying the second layered strip, and at least one third supply step for supplying the at least one third raw base material are provided, (f) a first partial shaping step is provided on the downstream side of the first supply step for shaping the first layered strip by heating and/or pressing and maintaining the shaped state, thereby producing a first preliminary shaped body, (g) a second partial shaping step is provided on the downstream side of the second supply step for shaping the second layered strip by heating and/or pressing and maintaining the shaped state, thereby producing a second preliminary shaped body, (h) a uniting step is provided on the downstream side of the first and second partial shaping steps and the at least one third supply step to unite the first and second preliminary shaped bodies (Pf1, Pf2) produced in the first and second partial shaping steps in such a way that the branching portion is formed in the preform and the at least one third raw base material is added at the branching portion formed, followed by uniting them by heating and/or pressing them in the configuration and maintaining the united state, thereby providing the preform, (i) a conveyance step is provided on the downstream side of the uniting step to intermittently convey the preform produced in the uniting step, and (j) the production of the first and second preliminary shaped bodies (Pf1, Pf2) in the first and second partial shaping steps and the production of the preform in the uniting step are carried out during the periods when the conveyance of the preform in the conveyance step is suspended.