In a joint assembly for moving a free end relative to a reference end like a human arm, first and second axle members have first and second axes associated with the reference and the free ends, respectively, and are coupled to each other by an axis end plane oblique to the first and the second axes. A coupling member, such as a universal joint, is coupled to the first and the second axle members and to the reference end. The first and the second axle members are rotated about the first and the second axes, respectively, to move the free end along a spherical surface, with the free end prohibited from being twisted. The oblique axis end plane may be provided by a pair of plate members or by a pair of shells. Alternatively, the axis end plane may be given by a stator and a rotor of a single motor. A motion of the free end is controlled by angles of rotation of the first and the second axle members.