A device for fixing biological soft tissue is endowed with strength and deformation performance for being used as a device for coupling biological soft tissue that has been cut or separated due to an incision or the like during a surgical procedure, and is completely degraded in vivo and discharged after adhesion of the soft tissue or after healing of the incision tissue. The device is composed of a ternary Mg alloy material of Mg—Ca—Zn. In the Mg alloy material, the Ca and Zn are contained within the solid-solubility limit with respect to the Mg. The remainder is composed of Mg and unavoidable impurities. The Zn content is 0.5 at % or less. The Ca and Zn content has a relationship of Ca:Zn=1:x (where x is 1 to 3) by atom ratio. The crystal grain structure is equiaxed, the crystal grain size according to linear intercept being 30 to 250 μm.