A light emitting device comprising: a light emitting structure including a first conductivity type semiconductor layer, an active layer on the first conductivity type semiconductor layer, and a second conductivity type semiconductor layer on the active layer; a buffer layer disposed under the light emitting structure; a light transmissive substrate disposed under the buffer layer and having a refractive index lower than a refractive index of the first conductivity type semiconductor layer; and a mirror structure layer disposed under the light transmissive substrate and including a first mirror layer having a first refractive index and a second mirror layer having a second refractive index different from the first refractive index, wherein the first mirror layer and the second mirror layer are alternately stacked on each other, wherein the first mirror layer has a thickness of W·λ·m/(4·n1), and the second mirror layer has a thickness of W·λ·m/(4·n2) in which λ represents a wavelength of a light emitted from the light emitting structure, n1 and n2 represent the first and second refractive indexes, respectively, m represents a natural number, and W represents a weight constant in a range of 1.05 to 1.25, wherein the first mirror layer is formed in plurality under the light transmissive substrate and has the first refractive index higher than a refractive index of the light transmissive substrate, and the second mirror layer is disposed between the plurality of first mirror layers and has the second refractive index lower than the first refractive index, wherein the mirror structure layer includes a pair of the first and second mirror layers (121, 122) repeatedly stacked on each other at least four times, wherein the first mirror layer or the second mirror layer makes contact with a lower surface of the light transmissive substrate, wherein at least one layer of the light emitting structure includes a compound semiconductor material having a compositional formula of In x Al y Ga 1-x-y N (0≤x≤1, 0≤y≤1, and 0≤x+y≤1), wherein a first light extraction structure is further formed on the lower surface of the light transmissive substrate, wherein the first light extraction structure is formed at a plurality of concavo-convex patterns on the lower surface of the light transmissive substrate, wherein at least one of the mirror layers (121, 122) of the mirror structure layer includes a concavo-convex layer by the first light extraction structure, wherein the light transmissive substrate includes a material having a refractive index lower than 2.4, wherein a difference between the first and second refractive indexes in the mirror structure layer is 1 or more, and wherein a difference in thicknesses between the first and second mirror layers (121, 122) is in a range of 50 nm to 70 nm. a light emitting structure including a first conductivity type semiconductor layer, an active layer on the first conductivity type semiconductor layer, and a second conductivity type semiconductor layer on the active layer; a buffer layer disposed under the light emitting structure; a light transmissive substrate disposed under the buffer layer and having a refractive index lower than a refractive index of the first conductivity type semiconductor layer; and a mirror structure layer disposed under the light transmissive substrate and including a first mirror layer having a first refractive index and a second mirror layer having a second refractive index different from the first refractive index, wherein the first mirror layer and the second mirror layer are alternately stacked on each other, wherein the first mirror layer has a thickness of W·λ·m/(4·n1), and the second mirror layer has a thickness of W·λ·m/(4·n2) in which λ represents a wavelength of a light emitted from the light emitting structure, n1 and n2 represent the first and second refractive indexes, respectively, m represents a natural number, and W represents a weight constant in a range of 1.05 to 1.25, wherein the first mirror layer is formed in plurality under the light transmissive substrate and has the first refractive index higher than a refractive index of the light transmissive substrate, and the second mirror layer is disposed between the plurality of first mirror layers and has the second refractive index lower than the first refractive index, wherein the mirror structure layer includes a pair of the first and second mirror layers (121, 122) repeatedly stacked on each other at least four times, wherein the first mirror layer or the second mirror layer makes contact with a lower surface of the light transmissive substrate, wherein at least one layer of the light emitting structure includes a compound semiconductor material having a compositional formula of In x Al y Ga 1-x-y N (0≤x≤1, 0≤y≤1, and 0≤x+y≤1), wherein a first light extraction structure is further formed on the lower surface of the light transmissive substrate, wherein the first light extraction structure is formed at a plurality of concavo-convex patterns on the lower surface of the light transmissive substrate, wherein at least one of the mirror layers (121, 122) of the mirror structure layer includes a concavo-convex layer by the first light extraction structure, wherein the light transmissive substrate includes a material having a refractive index lower than 2.4, wherein a difference between the first and second refractive indexes in the mirror structure layer is 1 or more, and wherein a difference in thicknesses between the first and second mirror layers (121, 122) is in a range of 50 nm to 70 nm.