An underwater pelletizing apparatus for realizing and stabilizing a pressure to press a cutting knife against a die plate, for preventing shaft vibration, and for decreasing wear of the cutting knife. The die plate and a water chamber casing are removably mounted at a thermoplastic material extrusion opening section; the cutting knife is disposed at the front of the die plate; a cutter shaft with the knife mounted at the forward end thereof is rotatably and axially movably supported in the water chamber casing and a shaft supporting housing to thereby move the cutter shaft forward and backward by means of a fluid pressure cylinder; and a locking means is provided for intermittently locking the axial movement of the cutter shaft.
