A process for preparing polyhydroxycarboxylic acid by conducting dehydration polycondensation of hydroxycarboxylic acids in the presence or absence of a catalyst and in the presence or absence of an organic solvent, comprising using a vertical high viscosity reactor during all or a portion of the dehydration polycondensation steps and conducting hot polycondensation while separating and removing low boiling components. The process can provide high molecular weight polyhydroxycarboxylic acid with ease and cheaply in industry.