A rotating electrical machine comprising: a plurality of core pieces (20; 20A; 20B; 120; 20C; 220) forming a stator core, each of the core pieces (20; 20A; 20B; 120; 20C; 220) comprising: contact surfaces (24, 26; 34, 36; 44, 46; 124, 126; 224, 226) along a radial direction relative to a rotation axis on one side end and another side end of the core piece (20; 20A; 20B; 120; 20C; 220) in a peripheral direction around the rotation axis; a tapered protrusion (23; 33; 43; 123; 223) on the contact surface (24; 34; 44; 124; 224) of the one side end of the core piece (20; 20A; 20B; 120; 20C; 220), the protrusion (23; 33; 43; 123; 223) comprising first inclined surfaces (23a, 23b; 33a, 33b; 43a, 43b; 123a, 123b; 223a, 223b) outside and inside in the radial direction; and a recess (25; 35; 45; 125; 225) on the contact surface (26; 36; 46; 126; 226) of the other side end of the core piece (20; 20A; 20B; 120; 20C; 220), the recess (25; 35; 45; 125; 225) comprising second inclined surfaces (25a, 25b; 35a, 35b; 45a, 45b; 125a, 125b; 225a, 225b) contacting the first inclined surfaces (23a, 23b; 33a, 33b; 43a, 43b; 123a, 123b; 223a, 223b) outside and inside in the radial direction to receive the protrusion (23; 33; 43; 123; 223) of adjacent core piece (20; 20A; 20B; 120; 20C; 220), a cross-sectional shape perpendicular to the rotation axis of the protrusion (23; 33; 43; 123; 223) is a shape such that, inside a circle (CA, CB) centered on a first end portion (A1, B1) of one side in the radial direction of the contact surface (24; 34; 44; 124; 224) with a distance (RA, RB) being a radius between a second end portion (A3, B3) on a tip side of the first inclined surface (23a, 23b; 33a, 33b; 43a, 43b; 123a, 123b; 223a, 223b) located on an opposite side of the first end portion (A1, B1) on the protrusion (23; 33; 43; 123; 223) and the first end portion (A1, B1), at least a part of the first inclined surfaces (23a, 23b; 33a, 33b; 43a, 43b; 123a, 123b; 223a, 223b) located on the opposite side of the first end portion (A1, B1) contact the second inclined surfaces (25a, 25b; 35a, 35b; 45a, 45b; 125a, 125b; 225a, 225b), characterized in that: the cross-sectional shape of the protrusion (23; 33; 43; 123; 223) is a shape such that a third end portion (A2, B2) on a base end side of the first inclined surface (23a, 23b; 33a, 33b; 43a, 43b; 123a, 123b; 223a, 223b) located on an opposite side of the first end portion (A1, B1) on the protrusion (23; 33; 43; 123; 223) is located on circumference of the circle (CA, CB). contact surfaces (24, 26; 34, 36; 44, 46; 124, 126; 224, 226) along a radial direction relative to a rotation axis on one side end and another side end of the core piece (20; 20A; 20B; 120; 20C; 220) in a peripheral direction around the rotation axis; a tapered protrusion (23; 33; 43; 123; 223) on the contact surface (24; 34; 44; 124; 224) of the one side end of the core piece (20; 20A; 20B; 120; 20C; 220), the protrusion (23; 33; 43; 123; 223) comprising first inclined surfaces (23a, 23b; 33a, 33b; 43a, 43b; 123a, 123b; 223a, 223b) outside and inside in the radial direction; and a recess (25; 35; 45; 125; 225) on the contact surface (26; 36; 46; 126; 226) of the other side end of the core piece (20; 20A; 20B; 120; 20C; 220), the recess (25; 35; 45; 125; 225) comprising second inclined surfaces (25a, 25b; 35a, 35b; 45a, 45b; 125a, 125b; 225a, 225b) contacting the first inclined surfaces (23a, 23b; 33a, 33b; 43a, 43b; 123a, 123b; 223a, 223b) outside and inside in the radial direction to receive the protrusion (23; 33; 43; 123; 223) of adjacent core piece (20; 20A; 20B; 120; 20C; 220), a cross-sectional shape perpendicular to the rotation axis of the protrusion (23; 33; 43; 123; 223) is a shape such that, inside a circle (CA, CB) centered on a first end portion (A1, B1) of one side in the radial direction of the contact surface (24; 34; 44; 124; 224) with a distance (RA, RB) being a radius between a second end portion (A3, B3) on a tip side of the first inclined surface (23a, 23b; 33a, 33b; 43a, 43b; 123a, 123b; 223a, 223b) located on an opposite side of the first end portion (A1, B1) on the protrusion (23; 33; 43; 123; 223) and the first end portion (A1, B1), at least a part of the first inclined surfaces (23a, 23b; 33a, 33b; 43a, 43b; 123a, 123b; 223a, 223b) located on the opposite side of the first end portion (A1, B1) contact the second inclined surfaces (25a, 25b; 35a, 35b; 45a, 45b; 125a, 125b; 225a, 225b), characterized in that: the cross-sectional shape of the protrusion (23; 33; 43; 123; 223) is a shape such that a third end portion (A2, B2) on a base end side of the first inclined surface (23a, 23b; 33a, 33b; 43a, 43b; 123a, 123b; 223a, 223b) located on an opposite side of the first end portion (A1, B1) on the protrusion (23; 33; 43; 123; 223) is located on circumference of the circle (CA, CB).