The present specification presents a technique for transmitting a PPDU by using a resource unit including carriers of different sizes. For example, a PPDU is transmitted by using five frequency bands, a left guard band, and a right guard band. Null subcarriers can be included in the five frequency bands according to the type of included resource unit. The null subcarriers can be the leftmost side or the rightmost side of each frequency band, and the number of included null subcarriers can be determined according to the size of the carrier included in each resource unit or the type of resource unit. A frequency band including DC carriers can include only one resource unit having a discontinuous carrier, and null subcarriers can be further included around the DC carriers.