A delay line structure includes a flat spiral delay line and two grounding guard traces. The flat spiral delay line is disposed in the layout layer in a manner of extending from an input end, bending clockwise inward until reaching a U-turn part, continuously extending and bending counterclockwise outward to an out put end so as to form two coupling areas, which are spiral and have an opening respectively. The two grounding guard traces are disposed in the layout layer in a manner of extending from the openings respectively toward the coupling areas, having an interval between the grounding guard traces and the flat spiral delay line, wherein the grounding guard traces close to the openings of the coupling areas are electrically connected to the grounding circuit through a via respectively.