A robot arm, comprising: a first arm, a second arm, an eccentric oscillating gear device configured as a a speed reducer for use in an articulated joint between the first arm and the second arm to transmit driving force while changing a rotation speed at a given rotation speed ratio, the speed reducer comprising a first cylindrical section adapted to be attachable to one of the first and second arms (20, 38), a second cylindrical section adapted to be attachable to a remaining one of the first and second arms (20, 38), a spur gear being rotated according to rotation of an input shaft, a crankshaft having an eccentric portion (46a, 46b) and being rotatable together with the spur gear, a gear member (48a, 48b) having a tooth portion and a through-hole in which the eccentric portion (46a, 46b) is inserted, one of the first and second cylindrical sections (12, 14) rotatably supports the crankshaft, and a remaining one of the first and second cylindrical sections (12, 14) has a tooth portion meshed with the tooth portion of the gear member (48a, 48b), the first and second cylindrical sections (12, 14) being adapted to be rotatable concentrically and relatively with respect to each other according to an oscillating movement of the gear member (48a, 48b) along with a rotation of the crankshaft, and the robot arm is characterized in that the robot arm is provided with a rotational phase adjusting means capable of adjusting rotational phase of the speed reducer so that the rotational phase of the speed reducer is set to be a given rotational phase when an orientation of the second arm with respect to an orientation of the first arm is set to a given orientation, the rotational phase adjusting means comprising a first reference portion for setting an orientation of the first cylindrical section to a given state with respect to the first arm, a second reference portion for setting an orientation of the second cylindrical section to a given state with respect to the second arm and a third reference portion for setting an orientation of the crankshaft to a given state on a basis of an orientation of the first cylindrical section and the second cylindrical section, wherein the first reference portion is provided in the first cylindrical section of the speed reducer at a position offset from an axis of the first cylindrical section, the second reference portion is provided in the second cylindrical section of the speed reducer at a position offset from an axis of the second cylindrical section and the third reference portion is provided in the crankshaft or the spur gear at a position offset from a crankshaft axis. a first arm, a second arm, an eccentric oscillating gear device configured as a a speed reducer for use in an articulated joint between the first arm and the second arm to transmit driving force while changing a rotation speed at a given rotation speed ratio, the speed reducer comprising a first cylindrical section adapted to be attachable to one of the first and second arms (20, 38), a second cylindrical section adapted to be attachable to a remaining one of the first and second arms (20, 38), a spur gear being rotated according to rotation of an input shaft, a crankshaft having an eccentric portion (46a, 46b) and being rotatable together with the spur gear, a gear member (48a, 48b) having a tooth portion and a through-hole in which the eccentric portion (46a, 46b) is inserted, one of the first and second cylindrical sections (12, 14) rotatably supports the crankshaft, and a remaining one of the first and second cylindrical sections (12, 14) has a tooth portion meshed with the tooth portion of the gear member (48a, 48b), the first and second cylindrical sections (12, 14) being adapted to be rotatable concentrically and relatively with respect to each other according to an oscillating movement of the gear member (48a, 48b) along with a rotation of the crankshaft, and the robot arm is characterized in that the robot arm is provided with a rotational phase adjusting means capable of adjusting rotational phase of the speed reducer so that the rotational phase of the speed reducer is set to be a given rotational phase when an orientation of the second arm with respect to an orientation of the first arm is set to a given orientation, the rotational phase adjusting means comprising a first reference portion for setting an orientation of the first cylindrical section to a given state with respect to the first arm, a second reference portion for setting an orientation of the second cylindrical section to a given state with respect to the second arm and a third reference portion for setting an orientation of the crankshaft to a given state on a basis of an orientation of the first cylindrical section and the second cylindrical section, wherein the first reference portion is provided in the first cylindrical section of the speed reducer at a position offset from an axis of the first cylindrical section, the second reference portion is provided in the second cylindrical section of the speed reducer at a position offset from an axis of the second cylindrical section and the third reference portion is provided in the crankshaft or the spur gear at a position offset from a crankshaft axis.