In Nagano Prefecture, the sudden death of many wild birds became appearent in 1997. In order to investigate the cause, the authors developed a simultaneous analytical method using GC/MS. The method was applied to investigate the reason of sudden wild bird death in 3 cases that happened in 1998. Therefore, the every cause was cleared as following. 1) In the case of Komagane City, seeds of buckwheat were found in the residues of their stomachs, and the buckwheat was subjected to analyses. A high concentration of highly toxic insecticide, Methomyl, was detected. The authors presumed that was the cause of birds' death. 2) In the case of Saku City, some flesh, probably from apples was found in the birds' stomachs. The result of flesh analysis was the highly toxic insecticide, EPN, detected in a high concentration. The authors assumed that was the cause of death. 3) In the case of Asashina Village, fish, shrimp, rice, among others were found in the birds' stomachs. The result of their analysis was the highly toxic insecticide, Benfuracarb, detected in a high concentration. The authors assumed that was the cause of death.