A tension obtaining unit obtains a measured tension value by measuring tension in sub-areas where tension should be obtained, which are set in a screen area to perforate of a stretched screen master. A control unit calculates tension in sub-areas where tension should not be obtained of the screen area to perforate as computed tension values by modifying the measured tension value, based on predefined information for tension computation. The control unit compares the measured tension value and computed tension values against information for determining perforation conditions stored in a storage unit and controls a screen making unit to execute perforation on determined perforation conditions fit for tension in the sub-areas where tension should be obtained and for tension in the sub-areas where tension should not be obtained.