A pneumatic servo bearing actuator for ultraprecise positioning consisting of a servo amplifier, a pneumatic servo valve and an aerostatic bearing is proposed as a new actuator construction in this study. As the first step of the study, this report presents a system representation of the actuator on the concept of lumped-parameter bearing model and four-terminal circuit theory, where the input parameters are the pressure controlled by a servo valve and the mass flow into the bearing clearance, while the output ones are the load capacity of the bearing (thrust force of the actuator) and the bearing gap (displacement of the actuator). Fundamental characteristics of the actuator - rated thrust force, stiffness, damping coefficient and dynamic response - are presented through the system representation, and its potential to realize an ultraprecise positioning is comprehensively discussed.