The present invention discloses a video decoding method performed by a video decoding device. The video decoding method according to an embodiment may include the steps of: obtaining NAL unit type information indicating a type of a current network abstraction layer (NAL) unit from a bitstream; and decoding, when the NAL unit type information indicates that the NAL unit type of the current NAL unit is an encoded data for an slice, the slice based on whether a mixed NAL unit type is applied to a current picture.