A primary object of the present invention is to provide a polylactic acid decomposition method that efficiently decomposes polylactic acid so that the polylactic acid can readily undergo degradation by a biological treatment such as methane fermentation. Specifically, the present invention provides a polylactic acid decomposition method involving a step of impregnating a polylactic acid-containing organic material with a treatment solution containing an organic acid salt and/or inorganic acid salt of an amine compound.