A horizontal rotary hook for a sewing machine includes a bobbin case holder accommodating a bobbin, a rotating hook made of a synthetic resin, and accommodating the bobbin case holder and rotated in a predetermined direction, the rotating hook including a peripheral wall and a peripheral end, a needle-thread passing opening formed in the peripheral wall of the rotating hook so as to be defined at least by a peripheral end of the peripheral wall, and a reinforcing rib provided on the peripheral wall of the rotating hook so as to be located in the vicinity of the peripheral needle-thread passing opening.