A method of manufacturing a spark plug which includes a center electrode extending in an axial direction (D1, D2); an insulator having an axial hole which extends in the axial direction (D1, D2) and in which the center electrode is disposed; a metallic shell disposed around the insulator ; and a ground electrode electrically connected to the metallic shell and forming a gap (g) between the ground electrode and the center electrode , the method comprising: an assembly step (S100) of forming an assembly by combining the metallic shell , the insulator , and the center electrode ; an application step (S110) of applying a voltage between the metallic shell and the center electrode of the assembly ; and a judgment step (S120, S140) of judging, in a period during which the voltage is applied, whether or not a discharge penetrating the insulator has occurred, by using an output value output from an optical sensor directed toward a forward end portion of the assembly ; wherein the insulator has an outer-diameter decreasing portion where the outer diameter of the insulator decreases toward the forward end side (D1) in the axial direction (D1, D2), and a large diameter portion provided on the rear end side (D2) of the outer-diameter decreasing portion ; the metallic shell has an inner-diameter decreasing portion where the inner diameter of the metallic shell decreases toward the forward end side (D1) in the axial direction (D1, D2); the assembly has a packing sandwiched between the outer-diameter decreasing portion and the inner-diameter decreasing portion ; a length of the insulator , measured in parallel to the axial direction (D1, D2), between a forward end of the metallic shell and a boundary between the large diameter portion and the outer-diameter decreasing portion falls within a range of 5 mm to 13 mm; and in the judgment step (S120, S140), the output value output by using a wavelength band of 320 nm or greater is used; and conditions for judging, in the judgment step (S120, S140), that a discharge penetrating the insulator has occurred, include a condition that the output value indicates that brightness is equal to or less than a predetermined threshold (S140, FIG. 9: conditions J1 to J3). an assembly step (S100) of forming an assembly by combining the metallic shell , the insulator , and the center electrode ; an application step (S110) of applying a voltage between the metallic shell and the center electrode of the assembly ; and a judgment step (S120, S140) of judging, in a period during which the voltage is applied, whether or not a discharge penetrating the insulator has occurred, by using an output value output from an optical sensor directed toward a forward end portion of the assembly ; wherein the insulator has an outer-diameter decreasing portion where the outer diameter of the insulator decreases toward the forward end side (D1) in the axial direction (D1, D2), and a large diameter portion provided on the rear end side (D2) of the outer-diameter decreasing portion ; the metallic shell has an inner-diameter decreasing portion where the inner diameter of the metallic shell decreases toward the forward end side (D1) in the axial direction (D1, D2); the assembly has a packing sandwiched between the outer-diameter decreasing portion and the inner-diameter decreasing portion ; a length of the insulator , measured in parallel to the axial direction (D1, D2), between a forward end of the metallic shell and a boundary between the large diameter portion and the outer-diameter decreasing portion falls within a range of 5 mm to 13 mm; and in the judgment step (S120, S140), the output value output by using a wavelength band of 320 nm or greater is used; and conditions for judging, in the judgment step (S120, S140), that a discharge penetrating the insulator has occurred, include a condition that the output value indicates that brightness is equal to or less than a predetermined threshold (S140, FIG. 9: conditions J1 to J3). <EOS>