PURPOSE: To prevent a fracture of a drill and to reuse it after grinding by detecting a displacement amount of an elastic foam body by a displacement sensor when it is perforated by a boring drill and determining the durable time limit when the displacement amount reaches the fixed value.

CONSTITUTION: Under the condition that a spot of a laser type displacement sensor 31 is matched to a measurement position M around the upper part of a boring drill insertion position, and under this condition, a zero signal is sent to a meter relay 33 from a sequencer 34. Then, a boring operation of a polyurethane foam body 10 by a drill 20 is started and displacement, of the foam body 10 in the measurement position M during the boring operation is detected continuously by the laser type displacement sensor 31 and its output signal is converted by a sensor amplifier 32 into a voltage signal to be outputted to the meter relay 33. When the detected voltage signal value is above the predetermined upper limit value of the displacement, an H signal is outputted, and an alarm buzzer 35a is rung when the H signals are outputted several times, for example three times, continuously, and then an automatic boring cycle is stopped.