Marker grooves of an optical fiber are formed by a marker groove forming device including at least, on a substrate, a fiber guide and optical fiber pressing springs formed on a side wall surface in the fiber guide. The optical fiber pressing springs include edges contacted and pressed to the side of an optical fiber stored in the fiber guide, and plate springs for pressing the edges to the side of the optical fiber with fulcra on the side wall surface in the fiber guide. The optical fiber is pressed to a side wall surface in the fiber guide, and the edges are formed at a predetermined distance from each other.