P300による虚偽検出の先行研究は,模擬犯罪と有罪知識検査(GKT)の期間が比較的短い。しかしながら,実務検査の約50%は,犯罪が行われてから少なくとも1ヶ月後に実施されている。したがって,本研究では,模擬犯罪から比較的長期間経過後における,P300によるGKTの検出可能性について被験者間デザインで検討した。参加者は直後群(10名),1ヶ月後群(10名),1年後群(10名)の3群に分けられた。すべての群は模擬犯罪とGKTの期間以外は全く同じ手続きである。呈示した視覚刺激は,5つの貴金属(イヤリング,ブローチ,指輪,ネックレス,時計)の写真であり,呈示持続時間300ms,刺激間間隔2500msでコンピュータの画面に呈示した。参加者には,各写真が画面に呈示されたら,できるだけ速くボタン押しをするように教示した。同時に,参加者は模擬犯罪で盗んだ品物を隠すように求められた。測定した指標は,刺激に対するP300(Fz,Cz,Pz)と反応時間である。結果として,裁決項目に対するP300振幅は,1年後群を除き,非裁決項目よりも有意に大きくなった。P300によるGKTは,模擬犯罪後,1ヶ月後でも有効であった。この結果は,P300を指標とした虚偽検出が,犯罪現場にも応用可能であることを示唆している。 : The previous experiments of detection of deception using P300 were the relatively short interval between a mock crime and the GKT examination. However, about 50% of the field polygraph tests were conducted at least one month after the criminal acts. In this study, therefore, we examined the effects of detecting possibility by the P300 GKT after a relatively long period from mock criminal acts with between-subjects design. Participants were divided into three groups : an immediate group (n=10), a one month later group (n=10) and a one year later group (n=10). All the groups were examined with the completely same procedure except interval between a mock crime and the GKT examination. The presented visual stimuli were five pictures of the precious items (earrings, brooch, ring, necklace, and watch) and each presented on a computer display with 300 ms duration and 2500 ms inter-stimulus interval. We asked the participants to push a button as soon as possible when each picture was presented on the computer display. At the same time, the participants were asked not to reveal the item they had stolen in the mock crime. The dependent measures were EEG-P300 (Fz, Cz, Pz) and reaction time to the stimuli. Results indicated that P300 amplitudes to the critical items were significantly larger than the amplitudes to the non-critical items except one year after group. The P300 GKT was effective even one month after the commission of the mock crime. This result suggests that field applications may be feasible with P300 as the dependent variable.