PURPOSE: To hold the whole reinforcing insulator at a desired temperature by preliminarily determining the distribution of calorific value of a heating element according to the distribution of thickness of a reinforcing insulator, measuring the temperature of the reinforcing insulator at least in one point, and controlling the total calorific value of a heating source to maintain the measured temperature constant.

CONSTITUTION: A heating element 13 is disposed on the outer circumference of an injection metal mold 12 in which the end parts of two cables 1a 1b to connect are received, and a temperature sensor 14 is buried in a position near the center of the cable connecting part of the injection metal mold 12. Then, an uncrosslinked insulating material 15 is poured into an injection metal mold 12. When a time is passed, the temperature reduction is increased in the connecting part center because of the thick insulating material 15 and reduced in the peripheral part in which the insulating material 15 is thin, and consequently, the temperature becomes nearly constant on the surfaces C of insulating layers 2a, 2b. Thus, the total energy quantity given to the heating element 13 is controlled by the temperature detected by the temperature sensor 14, whereby proper temperature distribution can be obtained on the surfaces C of the insulating layers 2a, 2b.