A spark plug (100, 100b, 100c, 100d, 100e) comprising: an insulator (10, 10c, 10d) having a through hole (12, 12c, 12d) extending in a direction of an axial line; a center electrode, at least a part of which is inserted into a leading end side of the through hole (12, 12c, 12d); a terminal metal fixture (40, 40c, 40d, 40e), at least a part of which is inserted into a rear end side of the through hole (12, 12c, 12d); and a connection portion (300, 300b, 300c, 300d, 300e) connecting the center electrode and the terminal metal fixture (40, 40c, 40d, 40e) together in the through hole, wherein the connection portion (300, 300b, 300c, 300d, 300e) includes a magnetic substance structure (200, 200b, 200d) including a magnetic substance (210, 210b) and a conductor (220, 220b), wherein the magnetic substance structure (200, 200b, 200d) contains: a conductive substance as the conductor (220, 220b); an iron-containing oxide as the magnetic substance (210, 210b); and a ceramic containing at least one of silicon (Si), boron, and phosphorous, wherein, in a cross-section of the magnetic substance structure (200, 200b, 200d) including the axial line, when a target region is defined as a rectangular region having the axial line as a center line, a side of 2.5 mm in a direction perpendicular to the axial line, and a side of 5.0 mm in the direction of the axial line, a region of the iron-containing oxide includes a plurality of grain-shaped regions in the target region, at least a part of an edge of each of the plurality of grain-shaped regions is covered with the conductive substance in the target region, and when a coverage is defined as a proportion of a length of a portion of the edge of the grain-shaped region covered with the conductive substance to an entire length of the edge of the grain-shaped region, an average value of the coverage of the plurality of grain-shaped regions is greater than or equal to 50% in the target region, characterized in that, in the target region in the cross-section of the magnetic substance structure (200, 200b, 200d), a total number of grain-shaped regions, an area of which is the same as an area of a circle with a diameter in a range of 400 μm or greater and 1,500 μm or less, is greater than or equal to 6. an insulator (10, 10c, 10d) having a through hole (12, 12c, 12d) extending in a direction of an axial line; a center electrode, at least a part of which is inserted into a leading end side of the through hole (12, 12c, 12d); a terminal metal fixture (40, 40c, 40d, 40e), at least a part of which is inserted into a rear end side of the through hole (12, 12c, 12d); and a connection portion (300, 300b, 300c, 300d, 300e) connecting the center electrode and the terminal metal fixture (40, 40c, 40d, 40e) together in the through hole, wherein the connection portion (300, 300b, 300c, 300d, 300e) includes a magnetic substance structure (200, 200b, 200d) including a magnetic substance (210, 210b) and a conductor (220, 220b), wherein the magnetic substance structure (200, 200b, 200d) contains: a conductive substance as the conductor (220, 220b); an iron-containing oxide as the magnetic substance (210, 210b); and a ceramic containing at least one of silicon (Si), boron, and phosphorous, a conductive substance as the conductor (220, 220b); an iron-containing oxide as the magnetic substance (210, 210b); and a ceramic containing at least one of silicon (Si), boron, and phosphorous, wherein, in a cross-section of the magnetic substance structure (200, 200b, 200d) including the axial line, when a target region is defined as a rectangular region having the axial line as a center line, a side of 2.5 mm in a direction perpendicular to the axial line, and a side of 5.0 mm in the direction of the axial line, a region of the iron-containing oxide includes a plurality of grain-shaped regions in the target region, at least a part of an edge of each of the plurality of grain-shaped regions is covered with the conductive substance in the target region, and when a coverage is defined as a proportion of a length of a portion of the edge of the grain-shaped region covered with the conductive substance to an entire length of the edge of the grain-shaped region, an average value of the coverage of the plurality of grain-shaped regions is greater than or equal to 50% in the target region, a region of the iron-containing oxide includes a plurality of grain-shaped regions in the target region, at least a part of an edge of each of the plurality of grain-shaped regions is covered with the conductive substance in the target region, and when a coverage is defined as a proportion of a length of a portion of the edge of the grain-shaped region covered with the conductive substance to an entire length of the edge of the grain-shaped region, an average value of the coverage of the plurality of grain-shaped regions is greater than or equal to 50% in the target region, characterized in that, in the target region in the cross-section of the magnetic substance structure (200, 200b, 200d), a total number of grain-shaped regions, an area of which is the same as an area of a circle with a diameter in a range of 400 μm or greater and 1,500 μm or less, is greater than or equal to 6.