PURPOSE: To cool an impedor core without allowing a cooling medium to remain in a pipe, by turning back the cooling medium introduced into a pipe to be subjected to high frequency welding and sucking and discharging the same together with gas to the outside of the pipe through the inside of a core case.

CONSTITUTION: Cooling liquid is fed from a liquid tank 1 through a flow rate regulator 2 into a pipe 12 under production by high frequency welding in the production direction 10 of the pipe. The cooling liquid 13 arriving at the part just before a shield 3 by passing the outside of an impedor core case 5 is sucked together with the gases 11, 14 in the pipe 12 from the front end of the case 5 by the suction effect of a vacuum pump 4 to cool an impedor core 6, whereafter the liquid is discharged, from the inside of the case 5 through an external piping 9, a filter 7 and a cooler 8, from the above-mentioned vacuum pump 4.