The purpose of the present invention is to provide a surface treatment agent composition which contains aluminum as the coating film formation component and exhibits excellent properties for removing tin ions within a treatment bath when subjecting a tin-plated steel to electrolytic surface treatment, thereby forming a coating film exhibiting excellent corrosion resistance and yellowing resistance. This surface treatment agent composition is used for subjecting a tin-plated steel or a tin-based alloy-plated steel to electrolytic surface treatment and contains aluminum ions, fluorine ions, and polycarboxylic acid. The tin ions that eluted from the tin-plated steel are precipitated within and removed from the treatment bath by using this surface treatment agent composition.