In a knife used in an underwater cutting granulating device for cutting and granulating a fused resin extruded from a die to a water chamber by a cutter rotated in the state where the knife is opposed to the die, the knife of the cutter has a convex guide surface for guiding a water flow in the direction of drawing it toward the die, which is formed on the surface opposite to the surface facing the die and a curved concave guide surface on the side facing the die.