PROBLEM TO BE SOLVED: To provide a joint mechanism with variable stiffness capable of being mounted on a robot arm, highly durable and capable of reducing influence by deterioration of an elastic member. SOLUTION: An arm comprises a forearm transmission mechanism 42 and a forearm model 3 and is provided on an upper arm 2 to be rotatable around a joint shaft 41a. The forearm transmission mechanism 42 has a center rail groove 42d and a side rail groove 42e extending in a longitudinal direction. The respective grooves 42d and 42e are provided at a predetermined distance in a vertical direction to the longitudinal direction of a model pipe 31 from the joint shaft 41a. Elastic members 43f have linearity and are provided on the upper arm 2 to be rotatable around the joint shaft 41a so that urging force is always directed to the shaft 41a. Each elastic member 43f is structured so that one end on the side of the joint shaft 41a slides along the respective grooves 42d and 42e to keep a distance between the other end and the shaft 41a constant when rotating around the shaft 41a. COPYRIGHT: (C)2009,JPO&INPIT