A flat-plate antenna having thinner shape and excellent productivity, reducing labor for installation in an electrical apparatus or on a wall, and exhibiting desired antenna characteristics stably is provided. The flat-plate antenna comprising a conductive flat-plate, a slit portion formed through said conductive plat-plate with width proportional to frequency band width, a radiating element portion disposed one side of said slit portion, a ground portion disposed other side of said slit portion, and a power supply line having a first conductor connected to said radiating element and a second conductor connected to said ground portion.