A synchronizing signal automatic selecting circuit comprising a signal inverter for inverting synchronizing signal from a synchronizing signal separating circuit, a synchronizing signal detector for detecting synchronizing signal when a synchronizing signal is inputted to a horizontal and horizontal/vertical synchronizing signal input terminal and grounding the output signal of the signal inverter, a polarity detecting circuit for detecting the synchronizing signal polarity of the horizontal and horizontal/vertical synchronizing signal input terminal and a vertical synchronizing signal separator for separating the vertical synchronizing signal from the output signal of a transistor. The circuit is designed to automatically select with priority the synchronizing signal where the composite video signal and the synchronizing signal are inputted at the same time to a monitor used for a peripheral equipment of a computer without using a switch.