The present invention relates to a built-in flash mechanism that will pop up when the illumination is insufficient or it is manually switched on. The present invention comprises a flash unit having a buckle, a driving motor, a driving gear set formed on said driving motor, a axle wheel portion having a clip, a driven gear set formed on said axle wheel portion, a light-sensitive sensor, and a pusher for obstructing light into said light-sensitive sensor. When the light is obstructs by the pusher, the sensor will activate the driving motor to release the flash device or lock the flash unit at a receiving chamber.