A GaN based light emitting device is provided to increase dimensions of an active layer per unit dimension by growing an active layer on an increased surface of a semiconductor layer having a rough surface. A first conductive type bottom GaN based semiconductor layer(13a) is formed on a sapphire substrate(11) in which a buffer layer(12) is formed. A MgN based compound semiconductor layer(14) is formed on the first conductive type bottom GaN based semiconductor layer. A first conductive type top GaN based semiconductor layer(13b) is formed on the MgN based compound semiconductor layer. An active layer(15) is formed on the first conductive type top GaN based semiconductor layer. A second conductive type GaN based semiconductor layer(16) is formed on the active layer.