A method of producing a polylactic acid block copolymer constituted of a segment(s) composed of L-lactic acid units and a segment(s) composed of D-lactic acid units, includes obtaining a mixture 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, the mixture having a weight average molecular weight of not less than 90,000 and a degree of stereocomplexation (Sc) satisfying: Sc=ΔHh/(ΔHl+ΔHh)×100>60 (wherein ΔHh represents the heat of fusion of stereocomplex crystals (J/g), and ΔHl represents the heat of fusion of crystals of poly-L-lactic acid alone and crystals of poly-D-lactic acid alone (J/g)); and subsequently subjecting the mixture to solid-phase polymerization at a temperature lower than the melting point of the mixture.