To prevent deformation and breakage of a support, and to form an insulator with accuracy.

A spark plug has an insulator 2, holding an axis hole 4 extended in a direction of an axis line CL1 for holding a center electrode 5 at a tip side in its axis hole 4, and a terminal electrode 6 on a rear end side in the axis hole 4. A manufacturing process of the insulator 2 includes a pressurized molding process, in which raw material powder PM is compressed and a molding body CP1, having a non-penetrated hole part HL serving as the axis hole 4 is molded; a support pin insertion process in which a rod-shaped support pin 61 is inserted from the rear-end side of the molding body CP1 into the hole part HL; and a grinding working process in which an outer peripheral face of the molding body CP1 with the support pin 61 inserted is ground so as to obtain an insulator intermediate IP which takes on an insulator shape. In the pressurized molding process, provided, a length of the molding body CP1 along an axis line CL1 is L, the molding body CP1 is molded so that a maximum grinding part MG, and a grinding volume with the grinding working process being a maximum exists within range of 2L/3 from the rear end of the molding body CP1.