Spark plug having improved adhesion between resistor and glass sealing layer

A spark plug comprising:

• a cylindrical metal shell;
• an insulator which has a through-hole formed inside the insulator along an axial direction of the metal shell and is held in the metal shell so as to be exposed from the metal shell;
• a center electrode which is inserted into and fixed to one end portion of the through-hole;
• a terminal metal piece which is inserted into and fixed to the other end portion of the through-hole;
• a resistor which is provided between the center electrode and the terminal metal piece in the through-hole and distant from the center electrode in the axial direction;
• a conductive glass sealing layer which is provided between the resistor and the center electrode in the through-hole without a gap;
• and
• a ground electrode which is electrically connected to the metal shell and configured such that a predetermined spark discharge gap is formed between its front end portion and the center electrode,
• wherein
• a diameter D of the conductive glass sealing layer is equal to or less than 3.3 mm,
• a joining surface of the conductive glass sealing layer and the resistor is a curved surface, an
• assuming that a surface area of the joining surface is Sa and an area of a cross-section of the conductive glass sealing layer, which is perpendicular to the axial direction and includes edge portions of the joining surface, is S1, Sa/S1 is equal to or greater than 1.1 and equal to or smaller than 3.0.