A field element core comprising: field magnet through holes (41, 42) that are arranged annularly in a circumferential direction around a given direction, said field magnet through holes being adjacent each other in said circumferential direction and forming a pair, each of said field magnet through holes having a pair of ends (411, 412, 421, 422) in said circumferential direction; and a connecting portion (11, 17, 18) provided between said field magnet through holes forming a same said pair, said connecting portion having, as sides (111, 112, 171, 172, 181, 182), said ends (412, 422) that belong to different said field magnet through holes and that are adjacent each other in said circumferential direction, and one (111, 171, 181) of said sides having at least one curved portion (111a, 121a, 131a, 141a, 151a, 161a, 171a, 181a, 181c) along a circle around a position (c111, c171, c1811, c1812) that is located, seen from said given direction, oppositely from said connecting portion with respect to said one of said sides, and wherein a tangent (t(r)) to said at least one curved portion seen from said given direction is along a first extension direction of extension of said connecting portion only at a certain one position (r13, r33, r73, r76) between both ends of said at least one curved portion, characterized in that: a radius (Rb) of said circle satisfies a ratio (Rb/Lm, Rb/Ln, Rb/Ln1, Rb/Ln2) of not less than 1.0 nor more than 1.5 with respect to a length (Lm, Ln, Ln1, Ln2) of said at least one curved portion in a direction perpendicular to a second extension direction (941, 942) in which one of said field magnet through holes extends from said connecting portion. field magnet through holes (41, 42) that are arranged annularly in a circumferential direction around a given direction, said field magnet through holes being adjacent each other in said circumferential direction and forming a pair, each of said field magnet through holes having a pair of ends (411, 412, 421, 422) in said circumferential direction; and a connecting portion (11, 17, 18) provided between said field magnet through holes forming a same said pair, said connecting portion having, as sides (111, 112, 171, 172, 181, 182), said ends (412, 422) that belong to different said field magnet through holes and that are adjacent each other in said circumferential direction, and one (111, 171, 181) of said sides having at least one curved portion (111a, 121a, 131a, 141a, 151a, 161a, 171a, 181a, 181c) along a circle around a position (c111, c171, c1811, c1812) that is located, seen from said given direction, oppositely from said connecting portion with respect to said one of said sides, and wherein a tangent (t(r)) to said at least one curved portion seen from said given direction is along a first extension direction of extension of said connecting portion only at a certain one position (r13, r33, r73, r76) between both ends of said at least one curved portion, characterized in that: a radius (Rb) of said circle satisfies a ratio (Rb/Lm, Rb/Ln, Rb/Ln1, Rb/Ln2) of not less than 1.0 nor more than 1.5 with respect to a length (Lm, Ln, Ln1, Ln2) of said at least one curved portion in a direction perpendicular to a second extension direction (941, 942) in which one of said field magnet through holes extends from said connecting portion.