A field element core, comprising: field magnet through holes (41, 42, 44) circularly disposed in a peripheral direction around a predetermined axis and being adjacent to each other in said peripheral direction to form sets, each of the field magnet through holes having a pair of ends (411, 412, 421, 422, 441, 442) in said peripheral direction; and a coupling part (11, 17, 18) provided between said field magnet through holes forming same said set, said coupling part having, as lateral surfaces (111, 171, 172, 181, 182), said ends (412, 442, 422, 441) that belong to said field magnet through holes different from each other and are adjacent to each other in said peripheral direction, wherein: at least one (111, 171, 181) of said lateral surfaces includes a curved portion (111a, 121a to 161a, 171a, 181a, 181c) extending along a circle with a position (c111, c171, c1811, c1812) being a center and positioned on a side opposite to said coupling part with respect to said lateral surface, viewed from a first direction along said axis; and a ratio x and a ratio y (Lc/Lm) satisfy: y ≤ x − 0.05; y ≥ − x + 1.05; y ≤ − 3 / 16 ⋅ x + 25 / 32; and y ≥ 3 / 16 ⋅ x + 7 / 32, wherein for said curved portion, x represents the ratio of the radius (Rb) of said circle with respect to a length (Lm, Ln, Ln1, Ln2) of the curved portion in a third direction perpendicular to a second direction (941, 942, 944) in which said field magnet through hole extends from said coupling part, and y represents the ratio of the distance (Lc) between a surface of said field magnet through hole positioned on the inner peripheral side in a radial direction and the center of said circle with respect to said length (Lm) of said coupling part in said third direction, wherein said second ratio (Lc/Lm) is a value other than 0.5, or wherein x does not fall within a range from 1.0 to 1.5, inclusive. field magnet through holes (41, 42, 44) circularly disposed in a peripheral direction around a predetermined axis and being adjacent to each other in said peripheral direction to form sets, each of the field magnet through holes having a pair of ends (411, 412, 421, 422, 441, 442) in said peripheral direction; and a coupling part (11, 17, 18) provided between said field magnet through holes forming same said set, said coupling part having, as lateral surfaces (111, 171, 172, 181, 182), said ends (412, 442, 422, 441) that belong to said field magnet through holes different from each other and are adjacent to each other in said peripheral direction, wherein: at least one (111, 171, 181) of said lateral surfaces includes a curved portion (111a, 121a to 161a, 171a, 181a, 181c) extending along a circle with a position (c111, c171, c1811, c1812) being a center and positioned on a side opposite to said coupling part with respect to said lateral surface, viewed from a first direction along said axis; and a ratio x and a ratio y (Lc/Lm) satisfy: y ≤ x − 0.05; y ≥ − x + 1.05; y ≤ − 3 / 16 ⋅ x + 25 / 32; and y ≥ 3 / 16 ⋅ x + 7 / 32, wherein for said curved portion, x represents the ratio of the radius (Rb) of said circle with respect to a length (Lm, Ln, Ln1, Ln2) of the curved portion in a third direction perpendicular to a second direction (941, 942, 944) in which said field magnet through hole extends from said coupling part, and y represents the ratio of the distance (Lc) between a surface of said field magnet through hole positioned on the inner peripheral side in a radial direction and the center of said circle with respect to said length (Lm) of said coupling part in said third direction, wherein said second ratio (Lc/Lm) is a value other than 0.5, or wherein x does not fall within a range from 1.0 to 1.5, inclusive. at least one (111, 171, 181) of said lateral surfaces includes a curved portion (111a, 121a to 161a, 171a, 181a, 181c) extending along a circle with a position (c111, c171, c1811, c1812) being a center and positioned on a side opposite to said coupling part with respect to said lateral surface, viewed from a first direction along said axis; and a ratio x and a ratio y (Lc/Lm) satisfy: y ≤ x − 0.05; y ≥ − x + 1.05; y ≤ − 3 / 16 ⋅ x + 25 / 32; and y ≥ 3 / 16 ⋅ x + 7 / 32, wherein for said curved portion, x represents the ratio of the radius (Rb) of said circle with respect to a length (Lm, Ln, Ln1, Ln2) of the curved portion in a third direction perpendicular to a second direction (941, 942, 944) in which said field magnet through hole extends from said coupling part, and y represents the ratio of the distance (Lc) between a surface of said field magnet through hole positioned on the inner peripheral side in a radial direction and the center of said circle with respect to said length (Lm) of said coupling part in said third direction, wherein said second ratio (Lc/Lm) is a value other than 0.5, or wherein x does not fall within a range from 1.0 to 1.5, inclusive.