AbstractA slot array antenna with parasitic slots was fabricated with the goal of increasing the gain of a slot antenna on a thick dielectric substrate operating in both millimeter and submillimeter wave regions. By using the design values obtained by microwave model experiments, a 94‐GHz millimeter‐wave slot array antenna was fabricated on a fused quartz substrate and the power gain of this antenna was experimentally studied. The fabricated antenna was found to operate as an array antenna at 94 GHz. Its power gain was increased by 5 ˜ 6 dB over that of the single‐slot antenna. This value agreed well with the 5.5‐dB gain improvement derived in the microwave model experiments. Hence, it was confined that the parasitic slots effectively contribute to the gain improvement of the antenna.