For applying a magnesium alloy to nails in the guided bone regeneration (GBR) method, sufficient strength and appropriate degradation speed are required to fix a membrane. In the case, the nails are exposed to body fluid after penetrating the alveolar bone. Therefore, in this research, degradation behavior after penetration of magnesium-calcium alloy which is expected to possess high biocompatibility was investigated. As a result, the Mg-Ca alloy nails were degraded inhomogeneously after immersion for 4 weeks in simulated body fluid. The degraded portions corresponded to the distribution of residual strain estimated by finite element analysis. Mg-Ca nails without precipitates possessed comparatively gradual degradation rate. It can be also confirmed that the region with residual strain degraded preferentially when compared the CT images and the residual strain distribution after penetration.