Spark plug

A spark plug comprising:

• an insulator having an axial hole extending therethrough in a direction of an axial line;
• a center electrode inserted into a forward end portion of the axial hole;
• a terminal electrode inserted into a rear end portion of the axial hole;
• and
• a resistor which is provided within the axial hole between the center electrode and the terminal electrode, said resistor containing an electrically conductive material and a glass containing silicon dioxide and boron oxide, wherein,
• a distance between the forward end of the terminal electrode and the rear end of the center electrode in the direction of the axial line is 15 mm or more;
• the glass is a phase-separated glass having aggregate phase containing silicon dioxide, and an intervening phase provided between the aggregate phase;
• the aggregate phase has a silicon dioxide content higher than that of the intervening phase;
• the intervening phase has a boron oxide content higher than that of the aggregate phase;
• and
• in a cross section of the resistor, the cross section including the axial line, and having a portion whose center corresponds to the axial line and which has a width of 1.3 mm in a direction perpendicular to the axial line, when a plurality of imaginary lines perpendicular to the axial line are drawn at intervals of 0.1 mm in the direction of the axial line, the number of aggregate phase located on each of the imaginary lines is determined, and the average number of aggregate phase per imaginary line is determined for each of a plurality of line groups each consisting of five consecutive imaginary lines, there are three or more consecutive line groups which satisfy the condition that the average number of aggregate phase per imaginary line is larger, by 5 or more, than the minimum average number of aggregate phase per imaginary line among the plurality of line groups.