In this invention, a decrease in insulator voltage endurance is suppressed when forming the insulator by using injection molding. The invention is a spark plug insulator production method comprising a forming step of forming a cylindrically shaped molded body having an axial hole that extends in the direction of an axial line by injection molding using a forming die having therein a column-shaped cavity and, disposed within the cavity, a rod-shaped member extending in the direction of the axial line. The forming step includes an injection step of injecting a material containing ceramics. In the injection step, the material is injected into the cavity through a plurality of injection ports open on the internal peripheral surface of the molding die forming the cavity. The plurality of injection ports include two or more injection ports whereof the positions in the direction of the axial line are different from one another, or, two or more injection ports whereof the positions in the peripheral direction are different from one another.