The ability of a continuum arm is influenced by the performance of an actuator. Although several extensible pneumatic actuators have been proposed in previous research, balancing strength with large strain is still a challenge. We propose an extensible pneumatic actuator with bellows (EPAB) that consists of a rubber tube and a highly packed bellows sleeve. We propose a parametric model of the EPAB that can comprise various bellows and rubber tubes. We measured the basic properties of four actuators with different parameters to verify the proposed model. We conducted evaluation experiments under two conditions: constant length with variable pressure and constant pressure with variable length. The experimental results indicate that the proposed model can roughly predict the measured data. The proposed extensible actuator has an unloaded strain of up to 350% at a pressure of 0.6 MPa.