To uniformly sharpen a large number of needle materials even if the needle materials are not orderly arranged when the needle materials are dipped into an electrolyte.

The tips of the needle materials 1 installed on a holding member 15 are arranged in order by melting the preceding needle materials during the period ranging from a time when a longest needle material 1A among the needle materials 1 installed on the holding member 15 reaches the liquid level R of the electrolyte to a time when the shortest needle material reaches the liquid level of the electrolyte. Then, the large number of needle materials 1 installed on the holding member 15 are dipped simultaneously into the electrolyte to uniformly melt all tips of the needle materials 1 so as to sharpen the needle materials.