To improve an interdental brush wire in penetrability into small spaces between teeth and fatigue strength by increasing the bending strength and rigidity of the wire, and making the wire smaller in diameter without reducing these factors.

A flat deformed wire of a track-shaped cross section with a width W and a thickness T is used as a twofold wire 3 of a clamp K in which a number of filaments 2 are clamped, twisted together and fixed in place. The substantial percentage of the cross-sectional area of the wire in the cross-sectional area of a circumscribed circle with a diameter D at the root portion Ka of the clamp K is set at at least 55% or more to increase the bending strength and rigidity of the clamp.