A rolling bearing unit comprising: a rolling bearing including an inner ring, an outer ring, and a plurality of rolling elements disposed between the inner ring and the outer ring; spacers including an inner ring spacer disposed adjacent to the inner ring and an outer ring spacer disposed adjacent to the outer ring; and a grease reservoir in which grease is reserved, and which is formed at a fixed ring spacer out of the inner ring spacer and the outer ring spacer, wherein the fixed ring spacer out of the inner ring spacer and the outer ring spacer has a first inner wall and a second inner wall that are opposed to each other in an axial direction of a shaft on which the rolling bearing is supported so as to define the grease reservoir, and the rolling bearing unit comprises a grease medium (23; 123A) being able to be impregnated with a base oil contained in the grease, the rolling bearing unit being characterized in that the grease medium (23; 123A) connects a fixed ring out of the inner ring when the inner ring spacer is the fixed ring spacer and the outer ring when the outer ring spacer is the fixed ring spacer to the second inner wall located so as to be opposed to the fixed ring in the axial direction, and is held between the second inner wall and the fixed ring out of the inner ring and the outer ring in a state where the fixed ring spacer out of the inner ring spacer and the outer ring spacer is disposed adjacent to the fixed ring out of the inner ring and the outer ring, wherein the grease medium (12; 123A) is positioned in the axial direction by positioning the fixed ring spacer with respect to the fixed ring. a rolling bearing including an inner ring, an outer ring, and a plurality of rolling elements disposed between the inner ring and the outer ring; spacers including an inner ring spacer disposed adjacent to the inner ring and an outer ring spacer disposed adjacent to the outer ring; and a grease reservoir in which grease is reserved, and which is formed at a fixed ring spacer out of the inner ring spacer and the outer ring spacer, wherein the fixed ring spacer out of the inner ring spacer and the outer ring spacer has a first inner wall and a second inner wall that are opposed to each other in an axial direction of a shaft on which the rolling bearing is supported so as to define the grease reservoir, and the rolling bearing unit comprises a grease medium (23; 123A) being able to be impregnated with a base oil contained in the grease, the rolling bearing unit being characterized in that the grease medium (23; 123A) connects a fixed ring out of the inner ring when the inner ring spacer is the fixed ring spacer and the outer ring when the outer ring spacer is the fixed ring spacer to the second inner wall located so as to be opposed to the fixed ring in the axial direction, and is held between the second inner wall and the fixed ring out of the inner ring and the outer ring in a state where the fixed ring spacer out of the inner ring spacer and the outer ring spacer is disposed adjacent to the fixed ring out of the inner ring and the outer ring, wherein the grease medium (12; 123A) is positioned in the axial direction by positioning the fixed ring spacer with respect to the fixed ring.