Provided is a method of manufacturing a microbially produced plastic having low coloration, good tone, and low odor, based on a simple, low-cost method that is suitable for industrial production. The method of manufacturing the microbially produced plastic is characterized in that a step (a) and a step (b), described below, are included: step (a) is a step of applying heat treatment to fat containing hydrogen peroxide, and step (b) is a step of culturing microbes in a culturing liquid containing the fat that has been subjected to the heat treatment in the step (a).