A method for manufacturing a prepreg, the method comprising step of dispersing a reinforcing fiber bundle to obtain a sheet, fabric, or web of reinforcing fiber base material, step of providing a binder to the reinforcing fiber base material produced in the step, and step of hybridizing a matrix resin composed of a thermoplastic resin with the reinforcing fiber base material provided with the binder produced in the step, wherein the steps to are carried out online and the prepreg is one such that the content of the reinforcing fiber bundle relative to the whole portion of the prepreg is from 10 to 80% by mass, the content of the binder relative to the whole portion of the prepreg is from 0.1 to 10% by mass, and the content of the matrix resin relative to the whole portion of the prepreg is from 10 to 80% by mass, wherein the step of dispersing a reinforcing fiber bundle to obtain a reinforcing fiber base material is any one of the following (a) to (c): (a) comprising step (i) of charging a reinforcing fiber bundle to a dispersion medium, step (ii) of preparing a slurry in which reinforcing fibers forming the reinforcing fiber bundle are dispersed in the dispersion medium, step (iii) of transporting the slurry, and step (iv) of removing the dispersion medium from the slurry to produce a papermaking base material containing reinforcing fibers, wherein C1/C2 is from 0.8 to 1.2 where the mass content of the reinforcing fibers in the slurry prepared in the step (ii) is expressed by C1, and the mass content of the reinforcing fibers in the slurry at the time of the commencement of the step (iv) is expressed by C2; (b) comprising step (i) of charging a reinforcing fiber bundle to a dispersion medium, step (ii) of preparing a slurry in which reinforcing fibers forming the reinforcing fiber bundle are dispersed in the dispersion medium, step (iii) of transporting the slurry, and step (iv) of removing the dispersion medium from the slurry to produce a papermaking base material containing reinforcing fibers, wherein the steps (i) to (iv) are carried out online and the level H1 of the surface of the slurry in the step (ii) is higher than the level H2 of the surface of the slurry in the step (iv); (c) comprising step (i) of charging a reinforcing fiber bundle to a dispersion medium, step (ii) of preparing a slurry in which reinforcing fibers forming the reinforcing fiber bundle are dispersed in the dispersion medium, step (iii) of transporting the slurry, and step (iv) of removing the dispersion medium from the slurry to produce a reinforcing fiber base material, wherein the steps (i) and (ii) are carried out in a dispersion vessel, the step (iv) is carried out in a papermaking vessel, the step (iii) is carried out in a transport portion that connects the dispersion vessel and the papermaking vessel, and the slurry is transported in a laminar flow state or in a transition region state from a laminar flow to a turbulent flow in the transport portion. (a) comprising step (i) of charging a reinforcing fiber bundle to a dispersion medium, step (ii) of preparing a slurry in which reinforcing fibers forming the reinforcing fiber bundle are dispersed in the dispersion medium, step (iii) of transporting the slurry, and step (iv) of removing the dispersion medium from the slurry to produce a papermaking base material containing reinforcing fibers, wherein C1/C2 is from 0.8 to 1.2 where the mass content of the reinforcing fibers in the slurry prepared in the step (ii) is expressed by C1, and the mass content of the reinforcing fibers in the slurry at the time of the commencement of the step (iv) is expressed by C2; (b) comprising step (i) of charging a reinforcing fiber bundle to a dispersion medium, step (ii) of preparing a slurry in which reinforcing fibers forming the reinforcing fiber bundle are dispersed in the dispersion medium, step (iii) of transporting the slurry, and step (iv) of removing the dispersion medium from the slurry to produce a papermaking base material containing reinforcing fibers, wherein the steps (i) to (iv) are carried out online and the level H1 of the surface of the slurry in the step (ii) is higher than the level H2 of the surface of the slurry in the step (iv); (c) comprising step (i) of charging a reinforcing fiber bundle to a dispersion medium, step (ii) of preparing a slurry in which reinforcing fibers forming the reinforcing fiber bundle are dispersed in the dispersion medium, step (iii) of transporting the slurry, and step (iv) of removing the dispersion medium from the slurry to produce a reinforcing fiber base material, wherein the steps (i) and (ii) are carried out in a dispersion vessel, the step (iv) is carried out in a papermaking vessel, the step (iii) is carried out in a transport portion that connects the dispersion vessel and the papermaking vessel, and the slurry is transported in a laminar flow state or in a transition region state from a laminar flow to a turbulent flow in the transport portion.