The present specification relates to: a wireless power receiving device, which can receive information about a power class on the basis of out-band communications at a short distance before the wireless power receiving device is positioned at the charging position of a wireless power transmitting device, and negotiate power class; the wireless power transmitting device; a wireless power transmitting method; a wireless power receiving method; and a wireless power transmitting system. According to the present specification, whether power of the wireless power transmitting device (or a charger) is suitable for the wireless power receiving device (or a device to be charged) is quickly identified remotely or at a short distance prior to a wireless power transmission (or wireless charging), appropriate charging power is set through negotiation, and then wireless charging can be safely performed.