A method for producing a long-length shaft having a cast hole extending in its longitudinal direction as a hollow structure, comprising: a preparation step of preparing a mold assembly in which a first split core mold, which is configured to form a casting surface of the cast hole, is fitted into a second split core mold, which is configured to form a peripheral wall portion forming at least a part of a peripheral wall of the cast hole, so that the first split core mold is fixed so as not to move relative to the second split core mold, and in which the second split core mold, into which the first split core mold is fitted, is then fitted in a main mold configured to form a portion of the shaft other than the peripheral wall portion and the casting surface of the cast hole so that the second split core mold is fixed so as not to move relative to the main mold; a casting step of pouring molten metal into the mold assembly so as to cast the shaft; a first turning step where the peripheral wall portion in the shaft obtained in the casting step is held by a holding device of a lathe so as to turn a predetermined portion of the shaft that is a part of a portion of the shaft other than the peripheral wall portion of the shaft by setting an outer surface of the peripheral wall portion as a reference surface; and a second turning step where the predetermined portion of the shaft that was turned during the first turning step is held by the holding device of the lathe so as to turn a portion other than the predetermined portion of the shaft by setting an outer surface of the predetermined portion as another reference surface, characterised in that the first split core mold comprises a body having a long columnar shape and a core portion protruding from said body, wherein a tip of the core portion comprises a through tenon, wherein the second split core mold is generally cylindrically formed so as to form the peripheral wall portion as generally cylindrical and comprises a tenon groove configured to correspond to the through tenon, and wherein the through tenon is inserted in the tenon groove such that the first split core mold is fixed so as not to move relative to the second split core mold. a preparation step of preparing a mold assembly in which a first split core mold, which is configured to form a casting surface of the cast hole, is fitted into a second split core mold, which is configured to form a peripheral wall portion forming at least a part of a peripheral wall of the cast hole, so that the first split core mold is fixed so as not to move relative to the second split core mold, and in which the second split core mold, into which the first split core mold is fitted, is then fitted in a main mold configured to form a portion of the shaft other than the peripheral wall portion and the casting surface of the cast hole so that the second split core mold is fixed so as not to move relative to the main mold; a casting step of pouring molten metal into the mold assembly so as to cast the shaft; a first turning step where the peripheral wall portion in the shaft obtained in the casting step is held by a holding device of a lathe so as to turn a predetermined portion of the shaft that is a part of a portion of the shaft other than the peripheral wall portion of the shaft by setting an outer surface of the peripheral wall portion as a reference surface; and a second turning step where the predetermined portion of the shaft that was turned during the first turning step is held by the holding device of the lathe so as to turn a portion other than the predetermined portion of the shaft by setting an outer surface of the predetermined portion as another reference surface, characterised in that the first split core mold comprises a body having a long columnar shape and a core portion protruding from said body, wherein a tip of the core portion comprises a through tenon, wherein the second split core mold is generally cylindrically formed so as to form the peripheral wall portion as generally cylindrical and comprises a tenon groove configured to correspond to the through tenon, and wherein the through tenon is inserted in the tenon groove such that the first split core mold is fixed so as not to move relative to the second split core mold.