A reactor (D1, D2, D3), comprising: an air-core coil (1, 6, 11, 51) formed by winding an elongated conductive member; and a core portion (2, 7, 52) that covers both ends and an outer circumference of said air-core coil (1, 6, 11, 51), wherein a ratio t/W of a length t of said elongated conductive member in a radial direction of said air-core coil (1, 6, 11, 51) to a length W of said elongated conductive member in an axial direction of said air-core coil (1, 6, 11, 51) is no more than 1, wherein one surface of said core portion (2, 7, 52) that opposes one end of said air-core coil (1, 6, 11, 51) and one other surface of said core portion (2, 7, 52) that opposes one other end of said air-core coil (1, 6, 11, 51) are parallel at least in regions covering the coil ends, wherein a circumferential direction surface of said elongated conductive member forming said air-core coil (1, 6, 11, 51) is perpendicular relative to the one surface of said core portion (2, 7, 52), characterized in that a ratio R/W of a radius R from a center to an outer circumference of said air-core coil (1, 6, 11, 51) to a length W of said elongated conductive member in the axial direction of said air-core coil (1, 6, 11, 51) is 2 to 4; and projections (h) protruding to said air-core coil (1, 6, 11, 51) are formed at positions, facing an air-core part (S1, S2) of said air-core coil (1, 6, 11, 51), on an upper face and a lower face of said core portion (2, 7, 52), said projections (h) being formed so as to satisfy: 0 < a ≤ W / 3 and r> √ A 2 + W / 2 2, wherein r is defined as the radius of said air-core part (S1, S2) of said air-core coil (1, 6, 11, 51), a is defined as the height from a core surface, opposing a coil end, of said projection (h), and A is defined as the radius of a projection bottom surface. an air-core coil (1, 6, 11, 51) formed by winding an elongated conductive member; and a core portion (2, 7, 52) that covers both ends and an outer circumference of said air-core coil (1, 6, 11, 51), wherein a ratio t/W of a length t of said elongated conductive member in a radial direction of said air-core coil (1, 6, 11, 51) to a length W of said elongated conductive member in an axial direction of said air-core coil (1, 6, 11, 51) is no more than 1, wherein one surface of said core portion (2, 7, 52) that opposes one end of said air-core coil (1, 6, 11, 51) and one other surface of said core portion (2, 7, 52) that opposes one other end of said air-core coil (1, 6, 11, 51) are parallel at least in regions covering the coil ends, wherein a circumferential direction surface of said elongated conductive member forming said air-core coil (1, 6, 11, 51) is perpendicular relative to the one surface of said core portion (2, 7, 52), characterized in that a ratio R/W of a radius R from a center to an outer circumference of said air-core coil (1, 6, 11, 51) to a length W of said elongated conductive member in the axial direction of said air-core coil (1, 6, 11, 51) is 2 to 4; and projections (h) protruding to said air-core coil (1, 6, 11, 51) are formed at positions, facing an air-core part (S1, S2) of said air-core coil (1, 6, 11, 51), on an upper face and a lower face of said core portion (2, 7, 52), said projections (h) being formed so as to satisfy: 0 < a ≤ W / 3 and r> √ A 2 + W / 2 2, wherein r is defined as the radius of said air-core part (S1, S2) of said air-core coil (1, 6, 11, 51), a is defined as the height from a core surface, opposing a coil end, of said projection (h), and A is defined as the radius of a projection bottom surface.