Recently micro soft actuators have been developed actively, because they have possibility to be applied to micro hands, catheters, and so on. Especially pneumatic rubber actuators have high potential because of simple structure and non-electrical drive leading high safety. In our previous research, the pneumatic rubber actuator that can bend into two directions by positive and negative pneumatic pressures have been developed. This paper reports miniaturization of the bending rubber actuator. The actuator is downsized from 2.0mm to 400μm in diameter. By nonlinear FEM, the actuator design is optimized, then by molding method, the actuator is fabricated. Using developed actuator of 400μm in diameter, experimentally the bi-directional bending motion by positive and negative pneumatic pressures have been confirmed.