A transmission device with a phase-adjustment function includes a first dielectric layer, a signal line, a ground plane, and a first parasitic element. The first dielectric layer has a first surface and a second surface which are opposite to each other. The signal line is disposed on the first surface of the first dielectric layer. The ground plane is disposed on the second surface of the first dielectric layer. The first parasitic element is coupled to a first connection point on the signal line. The first parasitic element is configured to provide a first delay phase.