The　design　of　a　sheet-beam　electron　gun　with　planar　cathode　was　made　with　the　help　of　a　three-dimensional　electrostatic　field　solver　that　was　capable　of　forming　a　sheet-beam　of　19　mA　at　12　kV.　It　uses　one-dimensional　three-fold　beam　cross-sectional　area　compression　meets　the　specific　requirement　of　a　beam　to　be　formed　of　height　30　A　mu　m　and　width　600　A　mu　m　at　the　beam-waist　position　with　over　100　A/cm(2)　uniform　current　density　and　0.068　pi-mm-mrad　emittance,　typically,　for　0.5　THz　devices.　A　novel　beam　focusing　electrode　(BFE)　provided　with　extended-corner　rectangular-aperture　geometry　alleviating　the　commonly　encountered　sheet-beam　formation　problem　with　a　gun　that　uses　a　conventional　BFE,　as　well　as　it　reduced　beam　emittance　more　than　50%.　The　practicability　of　the　design　was　tested　by　the　high-voltage,　the　thermal　and　structural　analyses.　Work　has　been　initiated　to　test　the　performance　of　special　high　current　scandate　cathode　using　anode-aperture　mapping.