Design approach to a planar unidirectional, complementary dipole antenna under quadruple mode resonance is proposed. The design initiates from a planar complementary dipole antenna, with a pair of flat electric dipoles to be closely incorporated with a slot element. As theoretically predicted by the equivalent multi-source model, quadruple mode can be successfully excited to yield unidirectional antenna with a height of about 0.04-wavelength. As have been numerically demonstrated and experimentally validated at the 2.5-GHz band, the antenna can be implemented on air substrate of 5 mm (i.e., 0.041-wavelength). The prototype antenna can exhibit a wide impedance bandwidth up to 31.5%, in which an available quadruple-mode bandwidth of 5.2% is attained: The antenna can exhibit a maximum front-to-back ratio up to 13.0 dB, with nearly equal radiation patterns in both E- and H-planes. As is concluded, the quadruple resonant mode has been successfully incorporated to a slot element and then to yield fully-integrable, unidirectional, planar complementary dipole antennas for the first time.