Light irradiation decreases critical current and increases voltage generated in flux flow state in Bi base oxide superconductor. The light of short wave length (<440nm) is effective for acceleration of S-N transition, although light absorption characteristics of specimen is independent of wave length. Voltage deviation observed in V-I characteristics drastically increases under light irradiation. Without light irradiation, even specimen temperature is increased by pressurization of liquid nitrogen, voltage deviation remains small. Magnetic field <200 G gives little change to light irradiation effect on V-I characteristics. However the voltage deviation decreases with magnetic field strength. Temperature rise due to light absorption seems not to be the main cause of acceleration of S-N transition, although it takes certain part. Light irradiation may help magnetic flux to escape from pinning center through electronic mechanism. Proposed model, in which light irradiation causes avalanche of flux and therefore large voltage generation, gives a qualitative explanation to the experimental results.