A grease composition includes base oil, a thickener, and an inorganic additive. The base oil is a poly-α-olefin having a base oil kinematic viscosity of 15 to 50 mm2/s at 40° C. The thickener is a diurea expressed by Structural Formula (1). The content of the thickener is 10 to 22 mass % with respect to the total amount of the base oil and the thickener. The inorganic additive is a mixture containing sepiolite and bentonite, and is organically modified. The content of the inorganic additive is 2 to 20 mass % with respect to the total amount of the base oil and the thickener. R1—NHCONH—R2—NHCONH—R3   (1)