This helically toothed belt power transmitting device (1) has: a toothed belt (10) having a plurality of teeth (12) tilted relative to the width direction of the belt; a drive pulley; and a driven pulley. The width of the helically toothed belt (10) is 1 mm to 20 mm, inclusive. The core (13) of the toothed belt (10) is a twisted cord containing high-strength glass fibers or carbon fibers and has a diameter of 0.2 mm to 0.9 mm, inclusive. The compressibility of the teeth (12) of the toothed belt (10) compressed by the grooves of the drive pulley and the driven pulley is 0% to 5%, inclusive.