A spark plug (10, 100, 200, 300, 400, 500, 600), comprising: a cylindrical metal shell having a front end to which a ground electrode is joined; an insulator formed with an axial hole and having an outer circumferential surface partially surrounded by the metal shell, the axial hole including a first hole portion and a second hole portion larger in inner diameter than the first hole portion and continuous to the first hole portion via a step portion; a center electrode (50, 650) having a rear end portion (51, 651) disposed on the step portion of the insulator and a leg portion extending from the rear end portion toward the ground electrode in an axis direction; a metal terminal having a front end portion disposed in the second hole portion with a space left between the front end portion of the metal terminal and the rear end portion (51, 651) of the center electrode (50, 650); a resistor (70, 170, 270, 370, 470, 570, 670) arranged between the front end portion of the metal terminal and the rear end portion (51, 651) of the center electrode (50, 650) within the second hole portion; and a conductive seal (80, 180, 280, 380, 480, 580, 680) brought into contact with the resistor (70, 170, 270, 370, 470, 570, 670) and the rear end portion (51, 651) of the center electrode (50, 650), wherein the conductive seal comprises a side-surface seal layer (81, 181, 281, 381, 481, 581, 681) being in contact with the whole of a side surface (57, 652) of the rear end portion (51, 651) and having a thickness of 10 μm or larger in a direction perpendicular to the axis direction; and, assuming that a projection area (59, 654) is defined by projecting the center July 2020 electrode (50, 650) onto the axial hole in the direction perpendicular to the axis direction around a center axis of the spark plug, a contact surface (71, 171, 271, 371, 471, 571, 671) of the resistor (70, 170, 270, 370, 470, 570, 670) brought into contact with the axial hole overlaps at least a part of the projection area (59, 654), characterised in that: the conductive seal (80, 180, 280, 380, 480, 580, 680) is formed of a composition containing particles of glass and particles of metal; and the specific resistance of the conductive seal is in the range between the specific resistance of the center electrode (50, 650) and the specific resistance of the resistor (70, 170, 270, 370, 470, 570, 670). a cylindrical metal shell having a front end to which a ground electrode is joined; an insulator formed with an axial hole and having an outer circumferential surface partially surrounded by the metal shell, the axial hole including a first hole portion and a second hole portion larger in inner diameter than the first hole portion and continuous to the first hole portion via a step portion; a center electrode (50, 650) having a rear end portion (51, 651) disposed on the step portion of the insulator and a leg portion extending from the rear end portion toward the ground electrode in an axis direction; a metal terminal having a front end portion disposed in the second hole portion with a space left between the front end portion of the metal terminal and the rear end portion (51, 651) of the center electrode (50, 650); a resistor (70, 170, 270, 370, 470, 570, 670) arranged between the front end portion of the metal terminal and the rear end portion (51, 651) of the center electrode (50, 650) within the second hole portion; and a conductive seal (80, 180, 280, 380, 480, 580, 680) brought into contact with the resistor (70, 170, 270, 370, 470, 570, 670) and the rear end portion (51, 651) of the center electrode (50, 650), wherein the conductive seal comprises a side-surface seal layer (81, 181, 281, 381, 481, 581, 681) being in contact with the whole of a side surface (57, 652) of the rear end portion (51, 651) and having a thickness of 10 μm or larger in a direction perpendicular to the axis direction; and, assuming that a projection area (59, 654) is defined by projecting the center July 2020 electrode (50, 650) onto the axial hole in the direction perpendicular to the axis direction around a center axis of the spark plug, a contact surface (71, 171, 271, 371, 471, 571, 671) of the resistor (70, 170, 270, 370, 470, 570, 670) brought into contact with the axial hole overlaps at least a part of the projection area (59, 654), characterised in that: the conductive seal (80, 180, 280, 380, 480, 580, 680) is formed of a composition containing particles of glass and particles of metal; and the specific resistance of the conductive seal is in the range between the specific resistance of the center electrode (50, 650) and the specific resistance of the resistor (70, 170, 270, 370, 470, 570, 670). the conductive seal (80, 180, 280, 380, 480, 580, 680) is formed of a composition containing particles of glass and particles of metal; and the specific resistance of the conductive seal is in the range between the specific resistance of the center electrode (50, 650) and the specific resistance of the resistor (70, 170, 270, 370, 470, 570, 670).