A bent pipe structure comprising: a pipe body that includes a bend, and a cross-sectional area enlargement portion that is formed by configuring a curved portion with a circular arc shape at an inner circumferential face on a bend direction inside of the bend so as to enlarge a cross-sectional area of a flow path of the pipe body; wherein the cross-sectional area enlargement portion includes a pair of side face portions that face each other along an orthogonal direction orthogonal to an axial line of the pipe body, and a bottom face portion that couples lower ends of the side face portions together, and, when viewed along the direction of the axial line, the cross-sectional area enlargement portion is formed in a curved shape that protrudes toward an outer circumferential side of the pipe body; characterised in that the bent pipe structure satisfies one of the following relationships, wherein P (mm) denotes the inner diameter of the pipe body, H (mm) denotes the separation distance between the pair of side face portions measured between end portions of the pair of side face portions along the orthogonal direction orthogonal to the axis line, as viewed along the direction of the axial line, and R (mm) denotes the radius of the curved portion: R = 1 when P = 6 and H = 1, R = 1 when P = 6 and H = 2, R = 1 or greater and R = 2 or less when P = 6 and H = 3, R = 1 when P = 6 and H = 4, R = 3 or greater and R = 5 or less when P = 16 and H = 5, R = 3 or greater and R = 5 or less when P = 16 and H = 7, R = 3 or greater and R = 7 or less when P = 16 and H = 9, R = 3 or greater and R = 7 or less when P = 16 and H = 11, R = 3 or greater and R = 5 or less when P = 16 and H = 13, R = 3 when P = 16 and H = 15, R = 7 when P = 23 and H = 8, R = 7 when P = 23 and H = 10, R = 7 when P = 23 and H = 12, R = 7 when P = 23 and H = 14, R = 7 or greater and R = 10 or less when P = 23 and H = 16, R = 7 when P = 23 and H = 18, and R = 7 when P = 23 and H = 20. a pipe body that includes a bend, and a cross-sectional area enlargement portion that is formed by configuring a curved portion with a circular arc shape at an inner circumferential face on a bend direction inside of the bend so as to enlarge a cross-sectional area of a flow path of the pipe body; wherein the cross-sectional area enlargement portion includes a pair of side face portions that face each other along an orthogonal direction orthogonal to an axial line of the pipe body, and a bottom face portion that couples lower ends of the side face portions together, and, when viewed along the direction of the axial line, the cross-sectional area enlargement portion is formed in a curved shape that protrudes toward an outer circumferential side of the pipe body; characterised in that the bent pipe structure satisfies one of the following relationships, wherein P (mm) denotes the inner diameter of the pipe body, H (mm) denotes the separation distance between the pair of side face portions measured between end portions of the pair of side face portions along the orthogonal direction orthogonal to the axis line, as viewed along the direction of the axial line, and R (mm) denotes the radius of the curved portion: R = 1 when P = 6 and H = 1, R = 1 when P = 6 and H = 2, R = 1 or greater and R = 2 or less when P = 6 and H = 3, R = 1 when P = 6 and H = 4, R = 3 or greater and R = 5 or less when P = 16 and H = 5, R = 3 or greater and R = 5 or less when P = 16 and H = 7, R = 3 or greater and R = 7 or less when P = 16 and H = 9, R = 3 or greater and R = 7 or less when P = 16 and H = 11, R = 3 or greater and R = 5 or less when P = 16 and H = 13, R = 3 when P = 16 and H = 15, R = 7 when P = 23 and H = 8, R = 7 when P = 23 and H = 10, R = 7 when P = 23 and H = 12, R = 7 when P = 23 and H = 14, R = 7 or greater and R = 10 or less when P = 23 and H = 16, R = 7 when P = 23 and H = 18, and R = 7 when P = 23 and H = 20. R = 1 when P = 6 and H = 1, R = 1 when P = 6 and H = 2, R = 1 or greater and R = 2 or less when P = 6 and H = 3, R = 1 when P = 6 and H = 4, R = 3 or greater and R = 5 or less when P = 16 and H = 5, R = 3 or greater and R = 5 or less when P = 16 and H = 7, R = 3 or greater and R = 7 or less when P = 16 and H = 9, R = 3 or greater and R = 7 or less when P = 16 and H = 11, R = 3 or greater and R = 5 or less when P = 16 and H = 13, R = 3 when P = 16 and H = 15, R = 7 when P = 23 and H = 8, R = 7 when P = 23 and H = 10, R = 7 when P = 23 and H = 12, R = 7 when P = 23 and H = 14, R = 7 or greater and R = 10 or less when P = 23 and H = 16, R = 7 when P = 23 and H = 18, and R = 7 when P = 23 and H = 20.