Disclosed is an injection unit having a rotatable device for preventing reverse flow. To connect first flow channels formed in a spacer and second flow channels formed in a screw head when the metering is performed, and prevent reverse flow of resin by cutting off the first flow channels and the second flow channels when the resin is injected, the injection unit has a torsion spring as spring means between a shaft of the screw head and a screw, thereby rapidly preventing movement of the melted resin by restoring force of the spring when the metering of the resin is finished.
