A PCDMR (photoconductivity detected magnetic resonance) method developed recently is applied to the study of a photoinduced electron-transfer reaction. The PCDMR spectrum of the transient radical-ion pair formed by the photochemical reaction of xanthone/N,N-diethylaniline in 2-propanol is observed for the first time. A peculiar PCDMR spectrum is observed by irradiation with strong microwaves. This spectrum is tentatively explained by spin-locking at the line center and by the transition at both broadened wing portions. The lifetime of the radical-ion pair is estimated to be 2 x 10(-7) s at room temperature and is anomalously long.