Microstrip-line-fed broadband aperture antennas were developed in the millimeter-wave band. We have developed broadband microstrip-to-waveguide transitions to connect a microstrip line and a waveguide. The waveguide transmission line was replaced by a radiating waveguide with an aperture to compose a microstrip-line-fed aperture antenna. Two types of aperture antennas were developed. First, the microstrip substrate is fixed between the two metal plates of a waveguide with an aperture and a back-short waveguide. Second, both the microstrip feeding-line and the back-short waveguide are accommodated in the two-layer LTCC substrate. Broadband performance was achieved due to the potential of the transition. The characteristics of the developed antennas were evaluated by simulations and experiments in the millimeter-wave band.