Blood pressure measurement is precisely performed by more correctly determining positions of a cuff and a heart without being affected by variations in physical features among subject persons and their postures during the measurement. The high-frequency voltage applied to the lower electrode is subjected to periodical on-off modulation, the on-off duty ratio of which is determined for each wafer or each plurality of wafers, to carry out plasma processing on the wafer. These string patterns can be defined by the grammar of regular expressions.