The purpose of the present invention is to provide a device for fixing biological soft tissue the device being composed of a magnesium based alloy material wherein the device 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. This device composed of a magnesium based alloy material 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.
