A slot array antenna with a cavity of a simple construction having a plurality of slots formed on a dielectric substrate and capable of providing good directional characteristics while minimizing inter-slots coupling. The slot array antenna (1) includes a power supply line in the form of a micro-strip line formed on one surface of the dielectric substrate, a slot antenna portion (10) with a plurality of slots (14a, 14b, 14c, 14d) formed on the other surface of the dielectric substrate in an array-like fashion so as to extend perpendicularly to the micro-strip line with the dielectric substrate interposed therebetween, and a reflector (40) with a cavity (42) having an opening disposed in opposition to a plane on which the plurality of slots (14a, 14b, 14c, 14d exist). In this case, if the cavity has a width narrow enough to cut off a frequency of radio waves to be used, the directionality of the slot array antenna 1 can be optimized.