PURPOSE: To improve water shutoff property by temporarily forming one side edge of a water shutoff tape of an overlapped portion in a condition of opening by a speci fied angle, heating here the upper side edge with an induction coil, and immediately sending the heated part to a sizing die to finish-form the part into a cylindrical shape.

CONSTITUTION: A projection formed on a side edge of a divided portion 1a by the setting of an L-shaped recess 6 has a vertical angle of 30°W45°. When a cable core 11 and a water shutoff tape 12 come into a sizing die 1 of a heating section, the water shutoff tape 12 is wrapped around the cable core 11 in the longitudinal direction and temporarily formed. At an overlapped portion of the tape, a tape edge on a divided portion 1b side is pressed with the projection of the divided portion 1a to be in an open condition by the angle of 30°W45° relative to the other edge. When a high fre quency current is applied to an induction coil 10, an induction current is produced in the overlapped portion of the water shutoff tape made of lead for example and Joule heat is produced with the current induced. While the heat is conducted to the tape surface and dissipated in the air, the heated portion is sent to a sizing die of a next stage while the tape surface is being kept at a temperature at which heat-melt bonding is possible so that the tape is formed into a cylindrical shape to constitute a water shutoff layer.