A flat display device includes, vertical synchronization lock means which generates an internal vertical synchronization signal, a window signal generating circuit which generates a window signal by use of the internal vertical synchronization signal, a detecting circuit which detects whether or not an external vertical synchronization signal is present in a period of the window signal, and a determination circuit which determines whether a preset condition that a plurality of detection signals are present in a preset period is satisfied or not and controls an output inhibition circuit to inhibit a gate signal from being output when the preset condition is not satisfied.