A flow path structure that connects together a plurality of flow paths each extending in a different direction, the flow path structure comprising: a joint body comprising a flow-path inner corner formed at a connecting section between one of the flow paths and another of the flow paths; and an adapter disposed adjacent to the flow-path inner corner so as to couple together the one of the flow paths and the other of the flow paths, at a curving curved flow-path inner wall face, wherein: the adapter includes a protrusion that projects toward a flow path inner side so as to form the curved flow-path inner wall face; a retainer portion is provided at the adapter; a retaining portion that engages with the retainer portion so as to retain the adapter inside the flow path is provided at a flow path inner face; the adapter includes a tubular section for insertion into the flow path; the protrusion is provided at one axial direction end side of the tubular section; the retainer portion is provided at an outer peripheral portion of the tubular section; characterized in that the retainer portion comprises a pair of protrusions that extend along the axial direction and are formed to an outer peripheral portion of the tubular section which forms a main body of the adapter; a retaining portion comprises a pair of grooves that extend along an axial line direction and are formed in a housing area of the joint body where the main body is inserted, and the protrusions of the adapter engage with the grooves. a joint body comprising a flow-path inner corner formed at a connecting section between one of the flow paths and another of the flow paths; and an adapter disposed adjacent to the flow-path inner corner so as to couple together the one of the flow paths and the other of the flow paths, at a curving curved flow-path inner wall face, wherein: the adapter includes a protrusion that projects toward a flow path inner side so as to form the curved flow-path inner wall face; a retainer portion is provided at the adapter; a retaining portion that engages with the retainer portion so as to retain the adapter inside the flow path is provided at a flow path inner face; the adapter includes a tubular section for insertion into the flow path; the protrusion is provided at one axial direction end side of the tubular section; the retainer portion is provided at an outer peripheral portion of the tubular section; characterized in that the retainer portion comprises a pair of protrusions that extend along the axial direction and are formed to an outer peripheral portion of the tubular section which forms a main body of the adapter; a retaining portion comprises a pair of grooves that extend along an axial line direction and are formed in a housing area of the joint body where the main body is inserted, and the protrusions of the adapter engage with the grooves. the adapter includes a protrusion that projects toward a flow path inner side so as to form the curved flow-path inner wall face; a retainer portion is provided at the adapter; a retaining portion that engages with the retainer portion so as to retain the adapter inside the flow path is provided at a flow path inner face; the adapter includes a tubular section for insertion into the flow path; the protrusion is provided at one axial direction end side of the tubular section; the retainer portion is provided at an outer peripheral portion of the tubular section; characterized in that the retainer portion comprises a pair of protrusions that extend along the axial direction and are formed to an outer peripheral portion of the tubular section which forms a main body of the adapter; a retaining portion comprises a pair of grooves that extend along an axial line direction and are formed in a housing area of the joint body where the main body is inserted, and the protrusions of the adapter engage with the grooves.