The invention provides a production method for a composite molded body having an undercut section. The method is able to readily produce a composite molded body having a complicated shape having an undercut section. The method comprises: a step (i) in which a non-impregnated precursor, containing a thermoplastic resin and carbon fibers having an average fiber length in the range of 1-100 mm, is heated and pressurized to produce an impregnated precursor; a step (ii) in which the impregnated precursor is heated to at least the melting temperature for the thermoplastic resin; a step (iii) in which the heated impregnated precursor is arranged inside a metal mold having an undercut mechanism; a step (iv) in which the undercut mechanism is clamped after or at the same time as being operated, and the impregnated precursor is pressurized; and a step (v) in which the metal mold is unclamped, the undercut mechanism is operated again, and the composite molded body is removed from the unclamped metal mold.