This study aims at making a small billet for micro-forging by extruding a sheet in the thickness direction. Experiments were carried out at the extrusion ratio of 2 by using a pure aluminum sheet of 2 mm in thickness and a billet of 1 mm in diameter was successfully formed on the sheet surface. The billet height varied depending on the constraining conditions and diameters of the specimen even at the same extrusion ratio and punch penetration. Difference in billet height was considered by dimensional changes of the specimen and the required force for billet formation. The highest billet was obtained when the specimen was completely constrained, otherwise the material flew in the unconstrained direction, resulting in decrease in billet height. The diameter of specimen affected the billet height in the case of no constraint in the lateral direction, in which higher billets were formed by using larger diameter specimens. These results were also explained by comparing the flow patterns estimated in finite element analysis. The billet heights estimated in the analysis agreed well with experimentally measured ones. In some cases, piping defects were observed at the bottom of the billet, which was prevented by keeping the punch penetration less than the critical value predicted in the simulation.