A method and an apparatus for transmitting a PPDU in a WLAN system are proposed. Specifically, a transmitter generates a PPDU and transmits the PPDU to a receiver. The PPDU includes a legacy field and an EHT field. The legacy field includes an L-STF. The EHT field includes a data field. The legacy field is configured on the basis of a CSD value per a transmission chain. The CSD value is determined by a candidate CSD value that is the minimum of the sums of a first absolute value and a second absolute value on the basis of a power ratio. The power ratio is a ratio of received power of the L-STF to received power of the data field. The first absolute value is an absolute value of a value related to 5 percent of a CDF relative to the power ratio. The second absolute value is an absolute value of a value related to 95 percent of the CDF relative to the power ratio.