A joint having a connection surface, comprising a main body including: a first connection surface and a second connection surface, each of which is structured to connect to a connection target such as a port of hydraulic equipment, a piping, and a joint, the connection target having a path for allowing a hydraulic oil to flow; a first opening end opened on the first connection surface and connecting to the path; first coupling holes each structured as a threaded hole, which are opened on the first connection surface and arranged on a circle concentric with the first opening end; a second opening end opened on the second connection surface and connecting to the path; second coupling holes each structured as a through hole or a threaded hole, which are opened on the second connection surface and arranged along a circle concentric with the second opening end; and a communication path communicating the first opening end with the second opening end, wherein: arrangement of the first coupling holes and that of the second coupling holes of the main body are made identical to each other, a first plane including the first connection surface of the main body is perpendicular to a second plane including the second connection surface, the second connection surface is a square with the second opening end as its center, and the first connection surface is a rectangle including a square identical to the foregoing square with the first opening end as the center of the square, characterized in that the rectangle has one side longer than a side of the square by a depth of the first coupling holes. a first connection surface and a second connection surface, each of which is structured to connect to a connection target such as a port of hydraulic equipment, a piping, and a joint, the connection target having a path for allowing a hydraulic oil to flow; a first opening end opened on the first connection surface and connecting to the path; first coupling holes each structured as a threaded hole, which are opened on the first connection surface and arranged on a circle concentric with the first opening end; a second opening end opened on the second connection surface and connecting to the path; second coupling holes each structured as a through hole or a threaded hole, which are opened on the second connection surface and arranged along a circle concentric with the second opening end; and a communication path communicating the first opening end with the second opening end, wherein: arrangement of the first coupling holes and that of the second coupling holes of the main body are made identical to each other, a first plane including the first connection surface of the main body is perpendicular to a second plane including the second connection surface, the second connection surface is a square with the second opening end as its center, and the first connection surface is a rectangle including a square identical to the foregoing square with the first opening end as the center of the square, arrangement of the first coupling holes and that of the second coupling holes of the main body are made identical to each other, a first plane including the first connection surface of the main body is perpendicular to a second plane including the second connection surface, the second connection surface is a square with the second opening end as its center, and the first connection surface is a rectangle including a square identical to the foregoing square with the first opening end as the center of the square, characterized in that the rectangle has one side longer than a side of the square by a depth of the first coupling holes.