Disclosed are a method and an apparatus for transmitting a data unit comprising guard intervals having different lengths. A method for transmitting a data unit in a wireless LAN comprises the steps of: a transmission STA generating a PPDU; and the transmission PPDU transmitting the PPDU to one or more reception STAs, wherein the PPDU sequentially comprises a first signal field, a second signal field, and a training field, wherein the first signal field is generated on the basis of a first IFFT size and is transmitted in a first OFDM symbol, the second signal field is generated on the basis of the first IFFT size and is transmitted in a second OFDM symbol, and the training field is generated on the basis of a second IFFT size and is transmitted in a third OFDM symbol.