A method of decoding a video supporting a plurality of layers, the method comprising: receiving information on a reference layer used for decoding a current picture for interlayer prediction; deriving a number of active reference layer pictures used for decoding the current picture based on the information on the reference layer; and performing interlayer prediction based on the number of active reference layer pictures, wherein when a layer identifier of a current layer comprising the current picture is not 0 and when a number of reference layer pictures available for interlayer prediction in the same access unit as that of the current picture is not 0, and when a reference layer picture available for interlayer prediction specified by values of maximum temporal sub-layer information on each layer and information on maximum allowed value of temporal sub-layer allowing inter-layer prediction in each layer among direct reference layer pictures comprised in all the direct reference layers of the current layer is present in the same access unit as that of the current picture and included in an interlayer reference picture set of the current picture, the number of active reference layer pictures is derived to be equal to the number of reference layer pictures, wherein the number of reference layer pictures is derived based on information indicating a number of direct reference layers of the current layer, the maximum temporal sub-layer information on each layer, the information on maximum allowed value of temporal sub-layer allowing inter-layer prediction in each layer and a temporal identifier of the current picture, and among the pictures in the direct reference layers of the current layer comprising the current picture, a picture in a reference layer is considered as a reference layer picture available for decoding the current picture for interlayer prediction when maximum temporal sub-layer information of the reference layer is greater than or equal to the temporal identifier of the current picture and when the temporal identifier of the current picture is 0, and wherein the active reference layer picture is a reference picture used for inter-layer prediction of the current layer, the direct reference layer is a reference layer that may be used for inter-layer prediction of the current layer, and the maximum temporal sub-layer information on a layer is the maximum number of temporal sub-layers for the layer. receiving information on a reference layer used for decoding a current picture for interlayer prediction; deriving a number of active reference layer pictures used for decoding the current picture based on the information on the reference layer; and performing interlayer prediction based on the number of active reference layer pictures, wherein when a layer identifier of a current layer comprising the current picture is not 0 and when a number of reference layer pictures available for interlayer prediction in the same access unit as that of the current picture is not 0, and when a reference layer picture available for interlayer prediction specified by values of maximum temporal sub-layer information on each layer and information on maximum allowed value of temporal sub-layer allowing inter-layer prediction in each layer among direct reference layer pictures comprised in all the direct reference layers of the current layer is present in the same access unit as that of the current picture and included in an interlayer reference picture set of the current picture, the number of active reference layer pictures is derived to be equal to the number of reference layer pictures, wherein the number of reference layer pictures is derived based on information indicating a number of direct reference layers of the current layer, the maximum temporal sub-layer information on each layer, the information on maximum allowed value of temporal sub-layer allowing inter-layer prediction in each layer and a temporal identifier of the current picture, and among the pictures in the direct reference layers of the current layer comprising the current picture, a picture in a reference layer is considered as a reference layer picture available for decoding the current picture for interlayer prediction when maximum temporal sub-layer information of the reference layer is greater than or equal to the temporal identifier of the current picture and when the temporal identifier of the current picture is 0, and wherein the active reference layer picture is a reference picture used for inter-layer prediction of the current layer, the direct reference layer is a reference layer that may be used for inter-layer prediction of the current layer, and the maximum temporal sub-layer information on a layer is the maximum number of temporal sub-layers for the layer.