Recently, many types of the linear actuator are studied and used to get the direct linear motion. This paper is concerned with the most simple linear actuator which is constructed with a horizontal rod-type iron core, a single-phase excitation coil and a moving aluminum ring on which the thrust is exerted. This is the same construction as the well-known jumping ring experiment system. To analyze the characteristics of this actuator, the most difficult point for analysis is how to obtain the formula to show the flux distribution because the magnetic circuit is opened. One new method used the infinite element method (FEM) is proposed for the reason that FEM is optimum to know the numerical flux distribution, but the basic method of analysis which was used in earlier papers is based on the equivalent circuit introduced by dual circuit theory. The calculated values of starting characteristics by this proposed method show close agreement with the experimental ones. This new method must be useful for the system having the open magnetic circuit. © 1996, The Institute of Electrical Engineers of Japan. All rights reserved.