IEEE802.11DCF is widely used wireless LAN standard. But, packet delivery ratio of broadcast transmission is much lower than unicast transmission because the IEEE802.11DCF executes broadcast transmission without RTS/CTS exchange and ACK process. Hence, many protocols have been proposed to improve reliability of the broadcast transmission. Most of these protocols are proposed based on collision recovery policy using ACK signal or NACK signal instead of ACK frame. Then, transmission overhead is increased by retransmission under the condition that network contains some hidden terminals since ACK or NACK signal could not prevent transmission of hidden terminals. This paper proposes a new broadcast protocol based on collision prevention policy using modified RTS/CTS exchange, and clarifies that our proposed protocol achieved highly reliability with computer simulations.