A spark plug insulator production method for manufacturing a tubular insulator for a spark plug having a large-diameter portion having the largest outside diameter, a portion smaller in outside diameter than the large-diameter portion, and an axial hole extending in a direction of an axial line, the method comprising the steps of: assembling a plurality of mold pieces (210, 220, 231, 232) to form an external mold; disposing a rod member in a space of a rear mold, the rear mold having an internal surface (18o, 11o) whose shape is identical to an external shape of a rear molded portion located rearward, in the direction of the axial line, of a molded portion corresponding to the large-diameter portion of the insulator, the internal surface (18o, 11o) defining the space, the rod member having an external surface identical in shape to a portion of the axial hole of the insulator corresponding to the rear molded portion; and injecting a material into a space (Sr) formed between the internal surface (18o, 11o) of the rear mold and the external surface of the rod member, thereby molding the rear molded portion; and releasing the rear molded portion (S230, S240) from the rear mold characterized in that the step of releasing the rear molded portion from the rear mold includes disassembling the rear mold into the plurality of mold pieces (231, 232) which form different portions of the internal surface (18o, 11o), the external mold for the spark plug insulator has a forward mold, an intermediate mold disposed rearward (Dfr) of the forward mold, and said rear mold disposed rearward (Dfr) of the intermediate mold, the plurality of mold pieces (231, 232) of the rear mold include a first mold piece which forms a first partial internal surface (18o1) of the internal surface (18o, 11o) of the rear mold along the entire circumference of the internal surface (18o, 11o), and a second mold piece which is located forward, in the direction of the axial line, of the first mold piece and forms a second partial internal surface (18o2, 11o) of the internal surface (18o, 11o) of the rear mold along the entire circumference of the internal surface (18o, 11o), and the step of releasing the rear molded portion from the rear mold includes starting to move the first mold piece rearward in the direction of the axial line in relation to the rear molded portion, and, subsequently to starting to move the first mold piece, starting to move the second mold piece rearward in relation to the rear molded portion, thereby disassembling the rear mold into the plurality of mold pieces (231, 232). assembling a plurality of mold pieces (210, 220, 231, 232) to form an external mold; disposing a rod member in a space of a rear mold, the rear mold having an internal surface (18o, 11o) whose shape is identical to an external shape of a rear molded portion located rearward, in the direction of the axial line, of a molded portion corresponding to the large-diameter portion of the insulator, the internal surface (18o, 11o) defining the space, the rod member having an external surface identical in shape to a portion of the axial hole of the insulator corresponding to the rear molded portion; and injecting a material into a space (Sr) formed between the internal surface (18o, 11o) of the rear mold and the external surface of the rod member, thereby molding the rear molded portion; and releasing the rear molded portion (S230, S240) from the rear mold characterized in that the step of releasing the rear molded portion from the rear mold includes disassembling the rear mold into the plurality of mold pieces (231, 232) which form different portions of the internal surface (18o, 11o), the external mold for the spark plug insulator has a forward mold, an intermediate mold disposed rearward (Dfr) of the forward mold, and said rear mold disposed rearward (Dfr) of the intermediate mold, the plurality of mold pieces (231, 232) of the rear mold include a first mold piece which forms a first partial internal surface (18o1) of the internal surface (18o, 11o) of the rear mold along the entire circumference of the internal surface (18o, 11o), and a second mold piece which is located forward, in the direction of the axial line, of the first mold piece and forms a second partial internal surface (18o2, 11o) of the internal surface (18o, 11o) of the rear mold along the entire circumference of the internal surface (18o, 11o), and the step of releasing the rear molded portion from the rear mold includes starting to move the first mold piece rearward in the direction of the axial line in relation to the rear molded portion, and, subsequently to starting to move the first mold piece, starting to move the second mold piece rearward in relation to the rear molded portion, thereby disassembling the rear mold into the plurality of mold pieces (231, 232).