A preform for the molding of a fiber-reinforced resin beam includes a reinforcing-fiber beam which has been formed from a reinforcing-fiber base made up of many reinforcing fibers and has a cross-sectional shape with at least one linear part and at least one curved part connected to the linear part; and a release sheet bonded and united through an adhesive resin to at least part of a surface of the reinforcing-fiber base along the lengthwise direction for the base. A process for producing a preform includes shaping a continuous flat reinforcing-fiber base having a release sheet bonded and united to a surface thereof so that the cross-sectional shape of the base has a curved part to thereby produce a preshaped object; and bonding and uniting the preshaped object to another preshaped object.