Method for producing a lost casting core having a side face (S1, S2) in a core box, which is composed of at least two core box parts (2, 3) between which a parting plane extends when the core box is closed, and which delimits a mould cavity, which determines the shape of the casting core to be produced, and in which an inner surface (14, 15) is provided, which maps the side surface (S1, S2) of the casting core, and through which the dividing plane of the core box runs, comprising the following working steps: (a) producing a casting core insert in one piece from a casting core moulding material, which is mixed from a mould base material and a binder and optionally from one or more additives, wherein the casting core insert corresponds to a portion of the casting core to be produced, and wherein the casting core insert bears at least one side surface portion (SF1, SF2), which is equal to a corresponding portion of the side surface of the casting core to be produced; (b) positioning the casting core insert in the mould cavity of the core box at a position in which the casting core insert occupies with its side surface portion (SF1,SF2) the space which the corresponding portion of the side face (S1, S2) of the casting core occupies in the finished casting core, wherein the dividing plane of the core box runs through the side face portion (SF1, SF2) of the casting core insert positioned in the core box; (c) introducing a casting core moulding material, which is mixed from a mould base material and a binder and optionally from one or more additives, into the mould cavity of the core box in order to produce the remaining casting core, wherein the casting core moulding material penetrating into the mould cavity comes into contact with the casting core insert seated in the mould cavity; (d) solidification of the casting core moulding material introduced into the mould cavity with formation of a form-fitting and/or material-fitting connection between the casting core moulding material introduced into the mould cavity and the casting core insert. (a) producing a casting core insert in one piece from a casting core moulding material, which is mixed from a mould base material and a binder and optionally from one or more additives, wherein the casting core insert corresponds to a portion of the casting core to be produced, and wherein the casting core insert bears at least one side surface portion (SF1, SF2), which is equal to a corresponding portion of the side surface of the casting core to be produced; (b) positioning the casting core insert in the mould cavity of the core box at a position in which the casting core insert occupies with its side surface portion (SF1,SF2) the space which the corresponding portion of the side face (S1, S2) of the casting core occupies in the finished casting core, wherein the dividing plane of the core box runs through the side face portion (SF1, SF2) of the casting core insert positioned in the core box; (c) introducing a casting core moulding material, which is mixed from a mould base material and a binder and optionally from one or more additives, into the mould cavity of the core box in order to produce the remaining casting core, wherein the casting core moulding material penetrating into the mould cavity comes into contact with the casting core insert seated in the mould cavity; (d) solidification of the casting core moulding material introduced into the mould cavity with formation of a form-fitting and/or material-fitting connection between the casting core moulding material introduced into the mould cavity and the casting core insert.