PURPOSE: To improve the flexing fatigue, water, and chemical resistance of a brush by containing polyester elastomer in a specified amount of high viscous polybutylen terephthalate.

CONSTITUTION: Monofilaments of 0.05W3.0mm in diameter for brushes made of the mixture of 50W90 in wt% of polybutylene terephthalate of which melt viscosity is 4,000 poises or more and 50W10 in wt% of polyester elastomer are formed. Constitution of a brush using these monofilaments as component parts improves the bent degradation, water, and chemical resistibilities of the brush. In this case, the polyester elastomer is a block copolymer composed of polyester hard segments and about 200W6,000 in number average monocular weight of polyether soft segments, and the ratio of the hard segments and soft segments is 15W90wt%, 85W10wt%.