A group proving method includes the following steps. First, a query command is broadcasted to radio frequency identification (RFID) tags, each of which responds the query command with tag identification data. Next, a first round parameter is generated and broadcasted according to the tag identification data, and each RFID tag responds the first round parameter with a first response parameter. Then, a second round parameter is generated and broadcasted according to the first response parameter, and each RFID tag determines whether the second round parameter is received in a predetermined period after the first round parameter was received and whether the first and second round parameters correspond to each other. If so, each RFID tag updates its tag key according to a random number parameter and outputs a second response parameter. Thereafter, group proving data are generated according to the above-mentioned data.