1. A synchronization-signal-generating device for supplying a synchronization signal to image pick-up devices of the type having image signal generation means for generating a picked up image signal of a pick-up frame-rate that is settable within a predetermined range and at a variable frame rate that changes the number of frames per unit time in the picked-up image signal to vary the frame rate of said picked up image signal as a function of frame rate information, thereby producing a frame-varied image signal, said synchronization-signal-generating device comprising: setting information generation means for generating frame rate information representing the image pick-up frame rate to which the picked up image signal is set and the number of frames per unit time of the picked up image said frame rate information being used to frame-synchronize the image signal generated by the image signal generation means of the image pick-up devices; synchronization signal generation means for generating the synchronization signal that corresponds to the frame rate information; synchronization signal output means for outputting the generated synchronization signal with the generated frame rate information being inserted thereinto; and control means for setting in said frame rate information the number of frames per unit time of the picked up image signal.