In recent years, attention has been focused on soft robots, such as robots with a high human affinity and robots that imitate the excellent movements of natural creatures. As actuators used in soft robots, structurally flexible soft actuators are required, not conventional motors or hydraulic/pneumatic cylinders. Various types of soft actuators have been developed depending on the driving principle. Pneumatic rubber artificial muscle is a kind of soft actuator that obtains power by injecting a working fluid such as air into an elastic structure such as rubber. The authors are developing the Straight fiber type artificial muscle as an actuator with excellent contraction characteristics. This artificial muscle has a structure in which a rubber tube contains reinforcing fibers arranged in the axial direction. When air pressure is applied to this, it expands only in the radial direction and contracts in the axial direction due to the restraint of the reinforcing fiber. While this artificial muscle has excellent contraction properties, it has a problem that it is difficult to enclose the reinforced fibers that have been gathered together in a rubber tube, which makes it difficult to manufacture. Therefore, in this study, as a material for pneumatic rubber artificial muscle, we investigated short fiber reinforced rubber in which aramid short fibers were oriented in one direction in the rubber. In the orientation direction of the short fibers, the tensile stress was 1.5 times and the elongation at break was 0.25 times that in the direction perpendicular to the orientation. It was confirmed that the artificial muscle made of short fiber reinforced rubber has a maximum contraction ratio of 17% and can be used as a rubber material for artificial muscle.