The effect of continuous light irradiation on the Briggs-Rauscher system has been investigated by the light intensity scanning method. A state diagram has been established in the control parameter plane spanned by the initial concentration of I -([I -] 0 ) and the light power (P). An increase of [I -] 0 leads to a transition from the large-amplitude oscillatory state (OS) to the reduced steady state (SSI), whereas an increase of P gives rise to a transition from OS to the oxidized steady state (SSII). Both transitions are accompanied by hysteresis, resulting in tristability among OS, SSI, and SSII in the overlapped part of the two hysteresis regions. In addition to the photoinhibition reported previously, the photoinduction of oscillations has been found in a certain range of [I -] 0 . In the low-[I -] 0 region, bistability between OS and another oscillatory state with small amplitude (OS′) has also been discovered. The origin of the transitions is discussed on the basis of photoinduced autocatalytic processes.