PURPOSE: To prevent strip miss and enable forecasting of repolishing time of a die by detecting the maximum load at the time of punching and also detecting the plunging force.

CONSTITUTION: By detecting the maximum load at the time of punching with a die in a press, the repolishing time of the die is forecasted and the damage in the edge of the die is discriminated. Also, by detecting the plunging force of the die, the prevention and inhibition of strip miss are discriminated. The sensor by which the maximum load and the plunging force are detected is composed of load cells 3. The detection signal of the load cells 3 is amplified by a strain amplifier 5, the output is inputted into an arithmetic processing unit (CPU) 7 and processed by a numerical controller 9. Consequently, the repolishing time of the die is surely detected with a simple structure.