PURPOSE: To prevent the material deterioration and lowering of material strength, by a method wherein, the resin located in the vicinity of electric heat generating body is molten by the electric heat generating body, while the molten resin is flowed to a duct inlet opened on the periphery of the joint by a generated stress, and variations in the periphery due to shrinkage of the joint are detected.

CONSTITUTION: A wire 6 for the measurement at the time of the expansion of plastic tube joint 1 at outer periphery is pulled out by the pulley of the displacement gage 7. When the outer periphery of the plastic tube joint 1 is shrunk momentarily by the release of the thermal stress, the wire 6 for measurement is rewound round the pulley of the displacement gage 7, which revolves, resulting in the generation of electric pulses by the pulse generator built in the displacement gage 7. The generated pulse is led to an arithmetic unit 8, discriminates the state of the electric pulses at the time point of the momentary shrinking, generates a signal to stop the power to be supplied to the electric heating resistor 2, by the function of the devices 9 which impresses power. Because of the above structure, the lowering of the strength of the joint part by the material deterioration caused by the over-heating does not occur. By the above consideration, the material deterioration, caused by the over-heating, at the part of the joint, together with the lowering of the material strength are prevented.