A liquid pipe coupling universal joint used in a pipe coupling portion, comprising: a first coupling structure having a first straight pipe portion and a first fitting portion as a curved body that forms a part of a spherical body connected to one end of the first straight pipe portion; and a second coupling structure having a second straight pipe portion and a second fitting portion as a curved body that forms a part of a spherical body connected to one end of the second straight pipe portion, wherein any one of the first and second fitting portions has a diameter larger than those of the first and second straight pipe portions and is fitted to the other fitting portion such that sphere centers of the first and second fitting portions match each other to allow semi-omnidirectional sliding or pivoting, the first and second straight pipe portions have center axis lines decentered from each other not to pass through sphere centers of the first and second fitting portions, a part of an inner surface (9, 10) of the first or second straight pipe portion and at least a part of an inner surface of the first or second fitting portion form a continuous plane portion as seen in a cross-sectional side view on a plane formed by the sphere center and the center axis line of the first or second straight pipe portion, and characterized in that an edge circumference of at least the first or second fitting portion is formed in a position separated from a reference plane by 10 to 30% of a maximum diameter length of the spherical portion formed on the reference plane in a direction opposite to a part where the straight pipe portion is connected, as seen in a cross-sectional side view on a first plane formed by the sphere center and the center axis line of the first or second straight pipe portion, the reference plane inside the sphere being formed on a second plane perpendicular to the first plane including a straight line having an angle of 30° to 68° with the center line portion as a straight line formed by linking the sphere center and the center line portion of the straight pipe portion. a first coupling structure having a first straight pipe portion and a first fitting portion as a curved body that forms a part of a spherical body connected to one end of the first straight pipe portion; and a second coupling structure having a second straight pipe portion and a second fitting portion as a curved body that forms a part of a spherical body connected to one end of the second straight pipe portion, wherein any one of the first and second fitting portions has a diameter larger than those of the first and second straight pipe portions and is fitted to the other fitting portion such that sphere centers of the first and second fitting portions match each other to allow semi-omnidirectional sliding or pivoting, the first and second straight pipe portions have center axis lines decentered from each other not to pass through sphere centers of the first and second fitting portions, a part of an inner surface (9, 10) of the first or second straight pipe portion and at least a part of an inner surface of the first or second fitting portion form a continuous plane portion as seen in a cross-sectional side view on a plane formed by the sphere center and the center axis line of the first or second straight pipe portion, and characterized in that an edge circumference of at least the first or second fitting portion is formed in a position separated from a reference plane by 10 to 30% of a maximum diameter length of the spherical portion formed on the reference plane in a direction opposite to a part where the straight pipe portion is connected, as seen in a cross-sectional side view on a first plane formed by the sphere center and the center axis line of the first or second straight pipe portion, the reference plane inside the sphere being formed on a second plane perpendicular to the first plane including a straight line having an angle of 30° to 68° with the center line portion as a straight line formed by linking the sphere center and the center line portion of the straight pipe portion.