PURPOSE: To obtain a sure biting detection signal of a tube material in piercing a tube material by a barrel-shaped roll and a plug at the tip of a mandrel bar by detecting compressive force acting on the mandrel bar.
CONSTITUTION: A mandrel bar 2 is connected to a thrust block 1 and a plug 3 is connected to the tip of the mandrel bar 2. A load detector 31 that detects compressive force that acts on the mandrel bar 2 is provided in the thrust block 1. In piercing a tube material 21 by a barrel-shaped roll 22 and a plug 3, a signal from the mandrel load detector 31 is compared with the set value of a load setting device 41. When a biting detector 42 detects that detected value of the detector 31 becomes larger than set value of the setting device 41, a signal 43 that indicates correct biting is outputted.