The goal of this study is to develop a miniaturized artificial muscle in which a tiny compressor can be installed. Pneumatic actuators, such as pneumatic artificial rubber muscles (PARMs), have been widely used in many industrial and robotic research applications because they are compact and lightweight. However, the compressors driving such actuators are relatively large. To solve this problem, the authors have been researching soft actuators driven by gas-liquid phase changes (GLPCs). In this study, a fixed chamber containing a constantan heater and fluorocarbon was used to generate pressure instead of a compressor. The pressure generation caused by the GLPC was confirmed, and a PARM contraction experiment was then conducted. Additionally, a PI control system was built to test the step and frequency responses of the actuator. A frequency response of up to 4.0 Hz was determined, and the corner frequency was found to be approximately 1.5 Hz. The size of the actuator was reduced by removing the chamber and installing the heater in the rubber muscle. A PARM driving experiment was conducted, and the performance of the PARM was evaluated. The miniaturized actuator consumes less power than the original actuator.