A waveguide microstrip line converter includes a waveguide, a dielectric substrate, a ground conductor including a slot, and a line conductor. The line conductor includes a first section that is a microstrip line having a first line width, a conversion unit that is a second section positioned immediately above the slot and having a second line width greater than the first line width, and a third section extending from the second section in a first direction and performing impedance matching between the first section and the second section. One of the opposite ends of the third section in the first direction is connected to the second section. The first section extends in a second direction perpendicular to the first direction continuously from the other end of the opposite ends of the third section.