PURPOSE: To provide the sail cloth of a toothed belt with self-stretchability so as to enhance its strength by using non-stretch yarn as warp in the sail cloth of the toothed belt and constraining the non-stretch yarn with plural stretch yarns.

CONSTITUTION: In stretch filament 15, two stretch yarns 16a, 16b are mutually twisted. Non-stretch yarn 17 is disposed in the zigzag state so as to pass though up and down successively between the plies of the stretch yarns 16a, 16n. Small loops 18 are formed at a small pitch on both sides of the stretch yarns 16a, 16b. The non-stretch yarn 17 is used as warp in the sail cloth 5 of a toothed belt 1. The strength of the sail cloth is thus enhanced, and abrasion resistance and durability are heightened.