A method of producing a composite molded body including joining a pre-molded preform (a) and a preform (b) with a fiber-reinforced thermoplastic resin by (1) providing reinforcing fibers having a weight average fiber length of 1 mm or more in at least one of the preforms (a) and (b), (2) using a thermoplastic resin (A) for the preform (a) and using the thermoplastic resin (A) or a thermoplastic resin (B) for the preform (b), (3) forming a thin film of a thermoplastic resin (C) on a surface of either the preform (a) or (b), or on surfaces of preforms (a) and (b), and (4) melting the thermoplastic resin (C) and a part of the preforms (a) and (b) by heating such that the thin film is placed at a boundary surface of a joint, and molding the composite molded body by the joint due to the melting.