Spiroplasma eriocheiris, a crustacean pathogen, is a helical-shaped swimming bacterium without a peptidoglycan layer. It swims in a high viscosity solution by switching the cell helicity from front to back. This swimming motility is driven by a unique intracellular structure, unlike the well-known bacterial flagellar motility. In this study, we analyzed the internal ribbon structure by electron microscopy to elucidate the mechanism of helicity switching. Negative staining and quick freeze, deep etch replica electron microscopy visualized that the helical ribbon structure has a half pitch of 350-nm. We isolated the ribbon structure by density gradient centrifugation and analyzed it by mass spectrometry. The ribbon was composed by bacterial actin MreBs and a Spiroplasma specific fibril protein. The protofilament obtained by disassembling the isolated ribbon structure was a double helix consisting of paired fibril filament. The half pitch of the protofilament corresponded to those of the cell helicity and the ribbon structure. The structure of the fibril filament was clarified at 4 - 5 angstrom resolution by electron cryomicroscopy and single particle analysis. The fibril filament has a left-handed long-pitch helicity, which is consistent with the 350-nm half pitch of the protofilament. Based on these results, we propose a model for the helicity switching in Spiroplasma swimming.