Since SIP uses a text-based message format and is open to the public Internet, it provides a number of potential opportunities for Denial of Service (DoS) attacks in a similar manner to most Internet applications. In this paper, we propose an effective detection method for SIP flooding attacks in order to deal with the problems of conventional schemes. We derive the upper bound of the possible number of SIP messages, considering not only the network congestion status but also the different properties of individual SIP messages such as INVITE, BYE and CANCEL. The proposed method can be easily extended to detect flooding attacks by other SIP messages.