A reactor, comprising: a core part provided with central leg parts (13A, 13B, 13F) and right and left leg parts (11A, 11B, 11F, 12A, 12B, 12F) arranged on both sides of the central leg parts (13A, 13B, 13F); a coil part (20, 20D) formed by winding a conducting wire around a circumference of the central leg parts (13A, 13B, 13F); and a heat transfer sheet (30, 30F) for dissipating heat in the coil part (20, 20D) to outside, wherein the coil part (20, 20D) is configured in such a manner that a rectangular wire is wound around the circumference of the central leg parts (13A, 13B, 13F) by edgewise winding and a circumference of the coil part (20, 20D) wound therearound is abutted on the heat transfer sheet (30, 30F), the coil part (20, 20D) is formed into a trapezoidal shape, in which a lower base on a side abutting on the heat transfer sheet (30, 30F) has a length as large as one and a half times or more a length of an upper base in a winding shape of one turn in the coil part (20, 20D), a minimum of interior angles is 60 degrees or more, and cross-sectional shapes of the central leg parts (13A, 13B, 13F) are configured to be trapezoidal shapes, the core part is formed by combining two substantially E-shaped partial cores (10A, 10B) in such a manner that leading end portions of three leg parts corresponding to each other are faced with each other, cross-sectional shapes of the right and left leg parts (11A, 11B, 11F, 12A, 12B, 12F) of the partial cores (10A, 10B) are formed into a shape so as to come along an outer shape of the coil part (20, 20D) wound, in a trapezoidal shape in a cross section, around a circumference of the central leg parts (13A, 13B, 13F), and the area of the leading end surface of each right and left leg part (11A, 11B, 11F, 12A, 12B, 12F) and the area of the cross section of the root portion of each central leg part (13A, 13B, 13F) are formed to be substantially equal to each other. a core part provided with central leg parts (13A, 13B, 13F) and right and left leg parts (11A, 11B, 11F, 12A, 12B, 12F) arranged on both sides of the central leg parts (13A, 13B, 13F); a coil part (20, 20D) formed by winding a conducting wire around a circumference of the central leg parts (13A, 13B, 13F); and a heat transfer sheet (30, 30F) for dissipating heat in the coil part (20, 20D) to outside, wherein the coil part (20, 20D) is configured in such a manner that a rectangular wire is wound around the circumference of the central leg parts (13A, 13B, 13F) by edgewise winding and a circumference of the coil part (20, 20D) wound therearound is abutted on the heat transfer sheet (30, 30F), the coil part (20, 20D) is formed into a trapezoidal shape, in which a lower base on a side abutting on the heat transfer sheet (30, 30F) has a length as large as one and a half times or more a length of an upper base in a winding shape of one turn in the coil part (20, 20D), a minimum of interior angles is 60 degrees or more, and cross-sectional shapes of the central leg parts (13A, 13B, 13F) are configured to be trapezoidal shapes, the core part is formed by combining two substantially E-shaped partial cores (10A, 10B) in such a manner that leading end portions of three leg parts corresponding to each other are faced with each other, cross-sectional shapes of the right and left leg parts (11A, 11B, 11F, 12A, 12B, 12F) of the partial cores (10A, 10B) are formed into a shape so as to come along an outer shape of the coil part (20, 20D) wound, in a trapezoidal shape in a cross section, around a circumference of the central leg parts (13A, 13B, 13F), and the area of the leading end surface of each right and left leg part (11A, 11B, 11F, 12A, 12B, 12F) and the area of the cross section of the root portion of each central leg part (13A, 13B, 13F) are formed to be substantially equal to each other.