Disclosed herein is a spacer including battery receiving parts formed at opposite major surfaces of a frame, respectively, the battery receiving parts having semi-cylindrical inner structures corresponding to outer surfaces of the cylindrical batteries, and assemblable coupling parts formed at opposite sides of the battery receiving parts, respectively, the assemblable coupling parts being engageable with and disengageable from the corresponding ones. The spacer according to the present invention has the effect of stably mounting cylindrical unit cells while preventing the occurrence of internal short circuits, manufacturing a structurally stable battery module, and effectively removing heat generated during charge of the cylindrical unit cells. Furthermore, the spacers can be easily assembled with each other depending upon the number of the unit cells required in manufacturing the battery module.