The present invention is a biodegradable magnesium alloy composed of x weight% of calcium, y weight% of zinc, balance of magnesium and unavoidable impurities, and x and y are represented by the following formula (1) (y = 44.894 × 2-25.123 × 5. 192) and the upper part of the trajectory according to Equation 2 (y = -10.618x2 + 7.8784x + 0.1637) have a range corresponding to a region where they overlap each other on the xy plane, and the zinc compound phase is a zinc compound phase. When the total weight of the phase is 90% by weight or more, it is characterized by containing Ca2Mg6Zn3.本発明は、x重量%のカルシウム、y重量%の亜鉛、残部のマグネシウム及び不可避不純物からなる生体分解性マグネシウム合金であり、xとyは、数式1(y=44.894x2-25.123x+5.192)による軌跡の下部と数式2(y=-10.618x2+7.8784x+0.1637)による軌跡の上部とがxy平面で互いに重畳される領域に該当する範囲を有し、亜鉛化合物相は、亜鉛化合物相を全体重量とするとき、90重量%以上のCa2Mg6Zn3を含むことを特徴とする。