A spark plug comprising: an insulator having an axial bore extending along an axis from a rear end of the insulator toward a front end of the insulator; a center electrode extending along the axis and having a rear end located inside the axial bore; a metal terminal including a trunk portion and a head portion, the trunk portion located inside the axial bore and having a front end located rearward of the rear end of the center electrode, the head portion located rearward of the trunk portion and exposed to the outside at the rear end of the insulator; and a conductive seal member in contact with the front end of the trunk portion of the metal terminal in the axial bore, wherein the insulator includes: a cylindrical first portion having a first inner diameter (Ra), the front end of the trunk portion of the metal terminal disposed therein; a cylindrical second portion having a second inner diameter (Rb) larger than the first inner diameter (Ra), and having a length of 1 mm or more forward of the rear end of the insulator, and a cylindrical third portion disposed between the first portion and the second portion, and having a rear end and a third inner diameter (Rc) larger than the first inner diameter (Ra) and smaller than the second inner diameter (Rb); and wherein the trunk portion of the metal terminal includes: a cylindrical front trunk portion, and a cylindrical rear trunk portion located rearward of the front trunk portion, the cylindrical rear trunk portion having a front end positioned forward of the rear end of the third portion of the insulator, the rear trunk portion having an outer diameter (Re) larger than an outer diameter (Rd) of the front trunk portion. an insulator having an axial bore extending along an axis from a rear end of the insulator toward a front end of the insulator; a center electrode extending along the axis and having a rear end located inside the axial bore; a metal terminal including a trunk portion and a head portion, the trunk portion located inside the axial bore and having a front end located rearward of the rear end of the center electrode, the head portion located rearward of the trunk portion and exposed to the outside at the rear end of the insulator; and a conductive seal member in contact with the front end of the trunk portion of the metal terminal in the axial bore, wherein the insulator includes: a cylindrical first portion having a first inner diameter (Ra), the front end of the trunk portion of the metal terminal disposed therein; a cylindrical second portion having a second inner diameter (Rb) larger than the first inner diameter (Ra), and having a length of 1 mm or more forward of the rear end of the insulator, and a cylindrical third portion disposed between the first portion and the second portion, and having a rear end and a third inner diameter (Rc) larger than the first inner diameter (Ra) and smaller than the second inner diameter (Rb); and a cylindrical first portion having a first inner diameter (Ra), the front end of the trunk portion of the metal terminal disposed therein; a cylindrical second portion having a second inner diameter (Rb) larger than the first inner diameter (Ra), and having a length of 1 mm or more forward of the rear end of the insulator, and a cylindrical third portion disposed between the first portion and the second portion, and having a rear end and a third inner diameter (Rc) larger than the first inner diameter (Ra) and smaller than the second inner diameter (Rb); and wherein the trunk portion of the metal terminal includes: a cylindrical front trunk portion, and a cylindrical rear trunk portion located rearward of the front trunk portion, the cylindrical rear trunk portion having a front end positioned forward of the rear end of the third portion of the insulator, the rear trunk portion having an outer diameter (Re) larger than an outer diameter (Rd) of the front trunk portion. a cylindrical front trunk portion, and a cylindrical rear trunk portion located rearward of the front trunk portion, the cylindrical rear trunk portion having a front end positioned forward of the rear end of the third portion of the insulator, the rear trunk portion having an outer diameter (Re) larger than an outer diameter (Rd) of the front trunk portion.