PURPOSE: To prolong the life of a press machine by measuring the load exerted on an upper die by a sensor, extracting only the prescribed frequency or below from the output thereof by a filter, subjecting the output to numerical pressing and displaying the minus load at the time of breakthrough.

CONSTITUTION: The detection output of a strain gage 6 mounted to the upper die of the press machine is inputted to the filter 22, by which the waveforms below the prescribed frequency are extracted. An A/D converter 24 can process a minus voltage as well and converts the analog signal from the filter 22 to a digital signal. A CPU block 30 samples the signal to the prescribed sampling frequency, stores the signal, makes spectral analysis thereof and outputs the load waveform including the minus waveform at the time of the breakthrough to a CRT display block 40. An operator is able to exactly and easily decide whether the punching press operation is adequate or not by viewing the waveform.