PROBLEM TO BE SOLVED: To provide a device for reliably tearing a seaweed in the longitudinal direction and reliably making thin the seaweed, so that the quality of products can be improved with a high processing speed to achieve an improved processing efficiency.SOLUTION: The device for tearing a slender stem seaweed W in the longitudinal direction so as to be processed into filaments includes: a clamp mechanism C which clamps one end of the seaweed W; a drum 10 having an outer peripheral surface 11 on which the other end of the clamped seaweed W is placed, which is to be rotated in the direction for causing the other end of the seaweed W to be separated away from the clamp mechanism C, and a rotary drive part 20 which rotates the drum 10. A plurality of needles N for tearing the seaweed W on the drum 10 by the rotation of the drum 10 are disposed in a row from the one side to the other side in the axial direction of the drum 10 on the outer peripheral surface 11 of the drum 10. The intervals of the needles N
