Herein disclosed is a robot arm mechanism comprising a first arm link mechanism and a second arm link mechanism, a robot arm driving mechanism for driving the first arm link mechanism and the second arm link mechanism, and a link retaining mechanism for pivotably retaining the first arm link mechanism and the second arm link mechanism, in which a third arm link and a fourth arm link of the first arm link mechanism are kept forward in a first rotation direction, in which the first arm link mechanism and the second arm link mechanism are extended, thereby enabling to prevent the quadric crank chain constituting the robot arm mechanism from being flattened out while the first arm link mechanism and the second arm link mechanism are extended, and improving resistance to deformation, in comparison with the conventional robot arm mechanism.