This disclosure provides a signal transmission communication unit and a coupler that can occupy a small area and have a reduced thickness. The signal transmission communication unit includes a base component including a signal transmission line and a ground electrode, a coupling planar conductor parallel to the base component and having a planar shape, an inductor circuit connected between the coupling planar conductor and the signal transmission line, and an LC-series circuits between part of the coupling planar conductor and the ground electrode and including a capacitor and an inductor connected in series. The inductor circuit is provided between the coupling planar conductor and the base component, and the LC-series connected circuit is provided between the coupling planar conductor and the base component.