A process for producing a polylactic acid block copolymer which has a high molecular weight, has a lower cost, and is capable of growing stereocomplex crystals only even when repeatedly heated/melted. The process for polylactic acid block copolymer production is characterized in that (i) D-lactide (ingredient D) is polymerized by ring-opening polymerization in the presence of poly-L-lactic acid (ingredient L) or (ii) L-lactide (ingredient L) is polymerized by ring-opening polymerization in the presence of poly-D-lactic acid (ingredient D) and that the ratio between the mass of the ingredient (D) and that of the ingredient (L) is such that ingredient (D)/ingredient (L) is from 60/40 to 91/9 or ingredient (L)/ingredient (D) is from 60/40 to 91/9.