The present invention relates to a device and a method for transmitting or receiving data in a wireless power transmission system. The present specification discloses a wireless power reception device comprising: a power pickup unit configured to receive, from a wireless power transmission device, wireless power generated on the basis of magnetic coupling in a power transmission phase; and a communication/control unit configured to receive, from the wireless power transmission device, information on a first buffer size indicating the size of a first buffer for receiving a data transmission stream. As the buffer size for exchanging the data transmission stream between the wireless power transmission device and reception device becomes clear, wireless power transmission and communication can proceed smoothly.