A spark plug comprising: a hollow metal shell having a first end and a second end opposed to the first end, said metal shell also having a threaded portion on an outer periphery thereof and an inner chamber opening at the first end, the threaded portion having an outer diameter equal to or less than 10 mm; an insulator having a length with a first end and a second end opposed to the first end of said insulator, said insulator also having a bore formed therein, said insulator being fixed in the inner chamber of said metal shell such that the first end of said insulator protrudes from the first end of said metal shell; a center electrode secured in the bore of said insulator, said center electrode having an end protruding from the first end of said insulator; a ground electrode having a side surface, said ground electrode being joined to the first end of said metal shell such that the side surface of said ground electrode is in opposed relationship with the end of said center electrode; a first noble metal chip having a first end joined to the end of center electrode, and a second end facing the side surface of said ground electrode; and a second noble metal chip having a first end joined to the side surface of said ground electrode and a second end facing the second end of said first noble metal chip, the second end of said second noble metal chip being spaced from the second end of said first noble metal chip so as to form a spark gap therebetween; wherein a surface area S of the second end of said first noble metal chip is in a range of 0.12 to 0.38 mm 2, inclusive; a length A of said first noble metal chip from the end of said center electrode to the second end of said first noble metal chip is in a range of 0.8 to 1.5 mm, inclusive; a surface area Q of the second end of said second noble metal chip is in a range of 0.12 to 0.65 mm 2, inclusive; a distance L between an inner surface of said metal shell defining the inner chamber and an outer surface of said insulator on a reference plane which extends perpendicular to the length of said insulator through an inner edge of the first end of said metal shell, is in a range of 1.2 to 1.6 mm, inclusive; and a space G of the spark gap between the second ends of the first and second noble metal chips (35, 45) is in a range of 0.4 to 1.0 mm, inclusive; characterized in that a length B of said second noble metal chip from the side surface of said ground electrode to the second end of said second noble metal chip is in a range of 0.5 to 1.2 mm, inclusive; and a thickness T of the insulator on the reference plane is in a range of 0.5 to 0.8 mm, inclusive. a hollow metal shell having a first end and a second end opposed to the first end, said metal shell also having a threaded portion on an outer periphery thereof and an inner chamber opening at the first end, the threaded portion having an outer diameter equal to or less than 10 mm; an insulator having a length with a first end and a second end opposed to the first end of said insulator, said insulator also having a bore formed therein, said insulator being fixed in the inner chamber of said metal shell such that the first end of said insulator protrudes from the first end of said metal shell; a center electrode secured in the bore of said insulator, said center electrode having an end protruding from the first end of said insulator; a ground electrode having a side surface, said ground electrode being joined to the first end of said metal shell such that the side surface of said ground electrode is in opposed relationship with the end of said center electrode; a first noble metal chip having a first end joined to the end of center electrode, and a second end facing the side surface of said ground electrode; and a second noble metal chip having a first end joined to the side surface of said ground electrode and a second end facing the second end of said first noble metal chip, the second end of said second noble metal chip being spaced from the second end of said first noble metal chip so as to form a spark gap therebetween; wherein a surface area S of the second end of said first noble metal chip is in a range of 0.12 to 0.38 mm 2, inclusive; a length A of said first noble metal chip from the end of said center electrode to the second end of said first noble metal chip is in a range of 0.8 to 1.5 mm, inclusive; a surface area Q of the second end of said second noble metal chip is in a range of 0.12 to 0.65 mm 2, inclusive; a distance L between an inner surface of said metal shell defining the inner chamber and an outer surface of said insulator on a reference plane which extends perpendicular to the length of said insulator through an inner edge of the first end of said metal shell, is in a range of 1.2 to 1.6 mm, inclusive; and a space G of the spark gap between the second ends of the first and second noble metal chips (35, 45) is in a range of 0.4 to 1.0 mm, inclusive; a length B of said second noble metal chip from the side surface of said ground electrode to the second end of said second noble metal chip is in a range of 0.5 to 1.2 mm, inclusive; and a thickness T of the insulator on the reference plane is in a range of 0.5 to 0.8 mm, inclusive.