We present a novel algorithm for physics-based real-time facial animation driven by muscle deformation. Unlike the previous works using 3D finite elements, we use a 2D shell element to avoid inefficient or undesired tessellation due to the thin structure of facial muscles. To simplify the analysis and achieve real-time performance, we adopt real-time thin shell simulation of [Choi et al. 2007]. Our facial system is composed of four layers of skin, subcutaneous layer, muscles, and skull, based on human facial anatomy. Skin and muscles are composed of shell elements, subcutaneous fatty tissue is assumed as a uniform elastic body, and the fixed part of facial muscles is handled by static position constraint. We control muscles to have stretch deformation using modal analysis and apply mass-spring force to skin mesh which is triggered by the muscle deformation. In our system, only the region of interest for skin can be affected by the muscle. To handle the coupled result of facial animation, we decouple the system according to the type of external forces applied to the skin. We show a series of real-time facial animation caused by selected major muscles that are relevant to expressive skin deformation. Our system has generality for importing new types of muscles and skin mesh when their shape or positions are changed.