To provide a method for melting bundles of bristles wherein each of edges of the bundles of the bristles inserted into the inside of a mold cavity is not expanded and cross sectional dimensions of each of molten parts formed in each of the edges of the bundles are not larger than those of each of the bundles.

In brush production by an in-mold forming method, a mold 1a provided with holes 2 for inserting the bundles of the bristles is used, wherein mold cavity the side a 3 has straight hole parts 2a and a remaining part of the mold has tapered hole parts 2b expanding toward the outside of the mold. Each of the bundles is inserted into each of the holes to extend inside the mold and then pressed back toward the outside of the mold, wherein the distance d1 between the edges of the bundles and the inside surface of the cavity is adjusted to be 0.5-1.5 mm for the bundles having a circumferential length of a cross section thereof with below 7 mm, to be 0.5-1.8 mm for that with 7-15 mm. Non-contact heating of the edges of the bundles by a heat source 7 after the adjustment forms molten parts 8 consisting of approximately the same sizes as the cross sections of the bundles. After the formation of the molten parts, the distance d2 between the edges of the molten parts and the inside of the cavity is adjusted to be a predetermined projection depth by moving the bundles again.