As a novel biodegradable metallic material the degradation speed of which in vivo can be controlled over a broad scope while achieving desired mechanical properties such as strength, work hardening and ductility without restricting the shape of an implant device, it is intended to provide a magnesium-based biodegradable metallic material which comprises Mg containing Mg as the major composition and having a concentration of inevitable impurities equal to or less than 0.05 atomic %, is free from precipitates or intermetallic compounds, and has an average grain size being regulated to equal to or less than 1/4 of the minimum part of the material.