To provide a thickener with which a grease excellent in high-temperature and high-speed durability can be produced, a grease containing the thickener, a method for producing the thickener and the grease, and a grease-sealed bearing in which the grease is sealed.

A thickener added to a base oil to obtain a grease contains an urea compound and an imide compound expressed by a specific formula. The imide compound is obtained by reacting an acid anhydride with an amino compound or by reacting an acid anhydride with an isocyanate compound. Examples of the imide compound include an imide compound obtained by reacting a diphenylmethane-4,4'-diisocyanate with a phthalic anhydride at a molar ratio of 1:2 in a base oil, an imide compound obtained by reacting a 4,4'-diaminodiphenyl ether with a phthalic anhydride at a molar ratio of 1:2 in a base oil, and an imide compound obtained by reacting a pyromellitic dianhydride with p-toluidine at a molar ratio of 1:2 in a base oil.