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, and which contains an electrically conductive material, and a glass containing silicon dioxide and boron oxide; wherein the glass is a phase-separated glass having aggregate phase grains containing silicon dioxide, and an intervening phase provided between the aggregate phase grains; the aggregate phase grains have a silicon dioxide content higher than that of the intervening phase; and the intervening phase has a boron oxide content higher than that of the aggregate phase grains; the spark plug being characterized in that 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; 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 (L 1,..., L n) 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 grains located on each of the imaginary lines is determined, and the average number of aggregate phase grains per imaginary line is determined for each of a plurality of line groups LG 1..., LG m) each consisting of five consecutive imaginary lines, there are three or more consecutive line groups (LG 11, LG m-10) which satisfy the condition that the average number of aggregate phase grains per imaginary line (L 11 -L 15, L n-14 -L n-10) is larger, by 5 or more, than the minimum average number of aggregate phase grains per imaginary line among the plurality of line groups. 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, and which contains an electrically conductive material, and a glass containing silicon dioxide and boron oxide; wherein the glass is a phase-separated glass having aggregate phase grains containing silicon dioxide, and an intervening phase provided between the aggregate phase grains; the aggregate phase grains have a silicon dioxide content higher than that of the intervening phase; and the intervening phase has a boron oxide content higher than that of the aggregate phase grains; the spark plug being characterized in that 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; 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 (L 1,..., L n) 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 grains located on each of the imaginary lines is determined, and the average number of aggregate phase grains per imaginary line is determined for each of a plurality of line groups LG 1..., LG m) each consisting of five consecutive imaginary lines, there are three or more consecutive line groups (LG 11, LG m-10) which satisfy the condition that the average number of aggregate phase grains per imaginary line (L 11 -L 15, L n-14 -L n-10) is larger, by 5 or more, than the minimum average number of aggregate phase grains per imaginary line among the plurality of line groups.