There is provided a joint mechanism constituted by a base 1, a turning base 4 provided above the base and rotated, and an arm 6 a lower end portion is pivotably supported by the turning base, in which a speed reducer 2 for rotating the turning base is provided at inside of the base, a counterbalance system 10 constituted by a cylinder 7 pivotably supported above the turning base and filled with a lubricant, a piston 8 provided in a hermetically sealed state slidably in a longitudinal direction at inside of the cylinder, and a piston rod 9 one end portion of which is connected to the piston and other end portion of which is connected to the arm 6 is provided, the speed reducer and the cylinder of the counterbalance system are connected and a lubricant of the speed reducer is introduced into the cylinder.