Spark plug

A spark plug comprising:

• a center electrode;
• a ceramic insulator which has an axial hole extending along an axis direction and holds the center electrode inside the axial hole;
• a metal shell which holds the ceramic insulator and surrounds a circumference of the ceramic insulator;
• a ground electrode, one end portion of which is fixedly connected with the metal shell, and the other end portion of which curves so that one side surface of the other end portion faces a top end portion of the center electrode;
• and
• an ignition portion which is provided at a position that faces the top end portion of the center electrode on the one side surface of the other end portion of the ground electrode and protrudes from the one side surface toward the center electrode, and
• the ignition portion having
• a pedestal portion which protrudes from the one side surface toward the center electrode;
• a discharge portion which is joined to a protrusion top end of the pedestal portion by laser welding and forms a spark discharge gap between the discharge portion and the top end portion of the center electrode, said discharge portion having an outside diameter of 0.7 mm or more;
• and
• a melting portion which intervenes between the pedestal portion and the discharge portion and is formed with constituent materials of both the pedestal portion and the discharge portion melting and mixed together by the laser welding,
• when viewing a chosen cross section of the ignition portion including a center axis of the ignition portion in a direction in which the ignition portion protrudes from the one side surface of the ground electrode, the melting portion being formed so as to extend from a side surface of the ignition portion toward the center axis,
• when viewing a profile line of the chosen cross section of the ignition portion, the melting portion having a configuration that connects a side surface of the pedestal portion and a side surface of the discharge portion, and
• in the chosen cross section of the ignition portion, X1 located in a boundary position between the pedestal portion and the melting portion at one of the side surfaces of the ignition portion, X2 located in a boundary position between the discharge portion and the melting portion at one of the side surfaces of the ignition portion, then when viewing a first cross section in which a distance of a straight line connecting the boundary positions X1 and X2 becomes a maximum in the chosen cross sections,
• a relationship between an outside diameter S and an extending length T satisfying T/S≧0.5, where S is the outside diameter of the discharge portion in a radial direction orthogonal to the center axis and where T is the extending length of the melting portion in a radially inward direction, on the basis of the boundary position X2 between the discharge portion and the melting portion, and
• an exterior angle θ formed between an imaginary line that passes through the boundary positions X1 and X2 and the center axis satisfying 135°≦θ≦175°.