An apparatus for continuously producing a tube having helical grooves in its inner surface comprising a tube twising rotary frame, a pay-off drum mounted on the rotary frame for paying off a tube having straight grooves in its inner surface, a nozzle provided at the front end of the rotary frame and having an adjustable inside diameter for sending out the paid-off tube forwardly of the rotary frame, and at least one pair of catching rollers provided in front of the rotary frame and disposed close to the nozzle for holding the sent-out tube. Each of the catching rollers is formed in its peripheral surface with a groove of semicircular cross section in conformity with approximately one-half of the outer periphery of the tube. With the rotation of the rotary frame, the straight-grooved tube is twisted between the nozzle and the catching rollers, whereby the straight grooves are deformed to helical grooves.