A method of promoting spheroid formation, including: a preparation step of preparing a mixture obtained by mixing a cell sample with a promoter; and a culture step of culturing, inside a spheroid formation-culture vessel, the mixture obtained in the preparation step, in which the promoter is a polymer in which one or more selected from the group consisting of D-glucosamine, D-galactosamine, D-glucuronic acid, L-iduronic acid, and D-galactose are polymerized.