Computers have been more widely used in recent years. Researchers are now able to analyze a magnetic field using the finite element method. A motor can be designed more effectively and rationally. It is important to understand design space. Design space means the limit of the thrust or design variables within constraints. Within these limited dimensions, a linear dc motor (LDM) for a pen recorder is designed. This LDM is very thin with long strokes. It is described as follows: (1) the thickness of the permanent magnet and the length of the gap are selected as the design variables in the magnetic circuit of the LDM; (2) the effective magnetic flux density of LDMs using the 3-D finite element method is compared; (3) the design space of thin LDM with long strokes is clarified. The LDM measurement value (thrust constant) was increased at 0.60 N/A from the first LDM. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 118 (4): 84–93, 1997