A spark plug comprising: a tubular metal shell; an insulator held within the metal shell and having a through hole formed therein so as to extend along a direction of an axial line of the metal shell; a center electrode inserted and fixed in first end portion of the through hole of the insulator; a metal terminal inserted and fixed in second end portion of the through hole of the insulator; a resistor disposed within the through hole and between the center electrode and the metal terminal so as to be spaced apart from the center electrode in the direction of the axial line; and a conductive glass seal layer provided within the through hole and between the resistor and the center electrode and electrically connecting the resistor and the center electrode to each other, wherein the conductive glass seal layer has a diameter of 3.9 mm or less, and a joined surface of the conductive glass seal layer and the resistor has a convex shape toward the center electrode side, characterized in that: when: α represents a length in the direction of the axial line from a rear end to a front end of the joined surface; and β represents a maximum length of the conductive glass seal layer in the direction of the axial line, a relation α/β ≥ 0.4 is satisfied, and in that: a shortest distance γ in the direction of the axial line from the joined surface to the center electrode is equal to or greater than 3 mm. a tubular metal shell; an insulator held within the metal shell and having a through hole formed therein so as to extend along a direction of an axial line of the metal shell; a center electrode inserted and fixed in first end portion of the through hole of the insulator; a metal terminal inserted and fixed in second end portion of the through hole of the insulator; a resistor disposed within the through hole and between the center electrode and the metal terminal so as to be spaced apart from the center electrode in the direction of the axial line; and a conductive glass seal layer provided within the through hole and between the resistor and the center electrode and electrically connecting the resistor and the center electrode to each other, wherein the conductive glass seal layer has a diameter of 3.9 mm or less, and a joined surface of the conductive glass seal layer and the resistor has a convex shape toward the center electrode side, characterized in that: when: α represents a length in the direction of the axial line from a rear end to a front end of the joined surface; and β represents a maximum length of the conductive glass seal layer in the direction of the axial line, a relation α/β ≥ 0.4 is satisfied, and in that: a shortest distance γ in the direction of the axial line from the joined surface to the center electrode is equal to or greater than 3 mm. when: α represents a length in the direction of the axial line from a rear end to a front end of the joined surface; and β represents a maximum length of the conductive glass seal layer in the direction of the axial line, a relation α/β ≥ 0.4 is satisfied, and in that: a shortest distance γ in the direction of the axial line from the joined surface to the center electrode is equal to or greater than 3 mm.