Method for promoting spheroid formation

Provided is a method for promoting spheroid formation, the method comprising a preparation step for preparing a mixture of a cell sample and a promoter, and a culture step for culturing the mixture obtained in the preparation step inside a vessel for spheroid formation and culture. The promoter is a polymer obtained by polymerization of one or more selected from the group consisting of d-glucosamine, d-galactosamine, d-glucuronic acid, l-iduronic acid, and d-galactose.