A polylactic acid block copolymer constituted by a poly-L-lactic acid segment(s) containing as a major component L-lactic acid and a poly-D-lactic acid segment(s) containing as a major component D-lactic acid, said polylactic acid block copolymer satisfying the Inequalities and below: Sc = ΔHh / ΔHl + ΔHh × 100 ≥ 80 wherein Sc is the degree of stereocomplexation [%], which means the ratio of stereocomplex crystals contained with respect to the total crystals in the polylactic acid, ΔHh represents the heat of fusion of stereocomplex crystals in DSC measurement wherein the temperature is increased at a heating rate of 20 °C/min. (J/g); and ΔHI represents the heat of fusion of crystals of poly-L-lactic acid alone and crystals of poly-D-lactic acid alone in DSC measurement wherein the temperature is increased at a heating rate of 20 °C/min. (J/g) 1 < Tm − Tms / Tme − Tm < 1.8 (wherein Tm represents the melting point derived from stereocomplex crystals observed at not less than 190 °C and less than 250 °C in DSC measurement wherein the temperature is increased at a heating rate of 40 °C/min.; Tms represents the beginning temperature of melting of stereocomplex crystals observed at not less than 190 °C and less than 250 °C in DSC measurement wherein the temperature is increased at a heating rate of 40 °C/min.; and Tme represents the end temperature of melting of stereocomplex crystals observed at not less than 190 °C and less than 250 °C in DSC measurement wherein the temperature is increased at a heating rate of 40 °C/min.), obtained by mixing poly-L-lactic acid and poly-D-lactic acid one of which has a weight average molecular weight of 60,000 to 300,000 and the other of which has a weight average molecular weight of 10,000 to 50,000, or mixing poly-L-lactic acid and poly-D-lactic acid having a ratio between the weight average molecular weight of poly-L-lactic acid and the weight average molecular weight of poly-D-lactic acid of not less than 2 and less than 30, to obtain a mixture having a weight average molecular weight of not less than 90,000 and satisfying the Inequality below: Sc = ΔHh / ΔHl + ΔHh × 100 > 60 to ≤ 99 wherein the weight average molecular weight is measured by gel permeation chromatography using hexafluoroisopropanol as a solvent and calculated in terms of poly(methyl methacrylate) standards, and Sc is the degree of stereocomplexation [%] of the kneaded mixture, which means the ratio of stereocomplex crystals contained with respect to the total crystals in the polylactic acid, ΔHh represents the heat of fusion of stereocomplex crystals in DSC measurement wherein the temperature is increased at a heating rate of 20 °C/min. (J/g); and ΔHI represents the heat of fusion of crystals of poly-L-lactic acid alone and crystals of poly-D-lactic acid alone in DSC measurement wherein the temperature is increased at a heating rate of 20 °C/min. (J/g), where poly-L-lactic acid and poly-D-lactic acid are melt-mixed at a temperature of not less than the end of melting point of the component having a higher melting point, followed by subjecting said mixture to solid-phase polymerization at a temperature lower than the melting point of said mixture. mixing poly-L-lactic acid and poly-D-lactic acid one of which has a weight average molecular weight of 60,000 to 300,000 and the other of which has a weight average molecular weight of 10,000 to 50,000, or mixing poly-L-lactic acid and poly-D-lactic acid having a ratio between the weight average molecular weight of poly-L-lactic acid and the weight average molecular weight of poly-D-lactic acid of not less than 2 and less than 30,