Problem to be solved. To provide highly safe PAI-1 inhibitors and compositions by identifying components derived from mackerel that suppress PAI-1 activity. Solution. The amount of peptides in an extract prepared from mackerel meat is measured, a diluent is prepared to a predetermined concentration, and the PAI-1 activity-inhibiting action is measured. With regard to a diluent exhibiting effective PAI-1 activity-inhibiting activity, fractionation based on molecular weight, polarity and hydrophilicity are carried out using high performance liquid chromatography (HPLC) analysis, and the PAI-1 activity-inhibiting action is measured on a fraction indicating a peak. Peptides as candidates for effective components are narrowed down, and two dipeptides (Gly-Lys, Pro-Lys) are identified as the effective components.