Glycolide production through depolymerization of glycolic acid oligomer is stabilized by controlling impurities in the glycolic acid oligomer to allow economical and effective production of glycolide. More specifically, the depolymerization system is controlled to suppress an effective concentration (x+100y wt. %) of depolymerization-obstructing substances calculated as a total of a concentration calculated as diglycolic acid (of x wt. %) of hydroxyl group-free impurity carboxylic acids (A) and 100 times a concentration calculated as ammonia (of y wt. %) of nitrogen-containing substances (B), respectively with respect to the starting glycolic acid oligomer in the depolymerization system, to at most 15 wt. %.