A mounting substrate includes a circuit board and a coupler. The circuit board has a mounting surface on which a microwave integrated circuit element is to be mounted, the microwave integrated circuit element having a plurality of terminals including first and second antenna I/O terminals. The coupler connects the first and second antenna I/O terminals to a waveguide device. The coupler includes: a first electrical conductor portion to be connected to the first antenna I/O terminal; a second electrical conductor portion to be connected to the second antenna I/O terminal; and an elongated gap in which an end face of the first electrical conductor portion and an end face of the second electrical conductor portion oppose each other. The elongated gap has a narrow portion at which the distance between the end face of the first electrical conductor portion and the end face of the second electrical conductor portion is locally decreased. The coupler couples an electromagnetic field being produced at the narrow portion to a waveguide in the waveguide device.