Abstract: A chuck mechanism for grasping a work comprises a stage, a master jaw provided on the stage, and a soft jaw fixed to the master jaw, in which a plurality of serrations extending in two different directions are formed on the abutting surfaces of master and soft jaws, respectively. Since the serrations are fitted, there is no such a risk that the soft jaw is displaced in the direction of the surface abutting against the master jaw. Consequently, reproducibility of fixing of the soft jaw becomes very high and a chuck mechanism having a soft jaw having a high reproducibility of fixing thereof can be provided.