A combination of a camera and a flash unit is provided with a control system for flash photographing. The control system comprises a plurality of signal generating circuits each for converting information to be transmitted between the flash unit and the camera into a signal of a predetermined level with a plurality of detecting circuits each associated with the corresponding signal generating circuits for detecting a signal of a detection level corresponding to a predetermined level, and the signal generating circuits and the associated detecting circuits forming pairs with one of each of the pairs being provided on the camera and the other being provided on the flash unit.