A trigger signal generating apparatus includes a frame synchronous circuit which receives a frame signal having a predetermined bit rate and outputs a synchronous signal in synchronism with an input timing of leading data of the frame signal, a position information output circuit which receives the synchronous signal output by the frame synchronous circuit and outputs the position information indicating an input bit position of the frame signal, a position designator which designates an arbitrary bit position of the frame signal, and a trigger signal generating circuit which outputs a trigger signal at a timing when the position information output by the position information output circuit is coincident with the arbitrary bit position designated by the position designator. A trigger is generated accurately at an arbitrary bit position of the frame position.