PURPOSE: To accurately detect the secondary machining proper temperature of a synthetic resin pipe by detecting the amount of displacement of a detection rod where an external force is applied after being in contact with the synthetic resin pipe.

CONSTITUTION: A far-infrared-ray heater 4 of an arm 3 which is fixed to a wall surface 11 of a heating oven 1 heats a synthetic resin pipe 2 from the inside and a far- infrared-ray heater 5 of a ceiling 12, a side wall, and a floor 14 heats the synthetic resin pipe 2 from the outside. When the pipe 2 is set to a heating position in the oven 1, an air cylinder 7 presses the pipe 2, and holds the pipe 2 between a roller 8 and a rotary roller 9, and the pipe 2 is uniformly heated after it is rotated in peripheral direction by the drive 9 of a motor 10. A detection rod 15 detects a position after a roller 16 at one end contacts the pipe 2 and follows the rotary surface of the pipe 2 with the load of a cylinder 17 whose the other edge is connected. When the pipe 2 is heated and the temperature reaches a secondary machining proper temperature, the rod 15 gradually moves and presses the pipe 2. Then, when the travel of a detection arm 18 reaches a certain value, a position sensor 19 converts the signal to an electrical signal. The powers of the heaters 4 and 5 are turned off by the electrical signal, thus completing heating.