A waveguide microstrip line converter includes a dielectric substrate, a ground conductor, and a line conductor. The ground conductor is provided on a first surface of the dielectric substrate and is joined to an open end that is an end portion of the waveguide. The slot is formed in a region surrounded by an opening edge portion of the open end of the ground conductor. The line conductor is provided on a second surface of the dielectric substrate. The line conductor includes first portions that are the microstrip lines, a second portion located just above the slot, and third portions responsible for impedance matching between the first portions and the second portion. The third portions each include an impedance transforming unit that is a portion having a wider line width than the first portions.