In wireless networks, data frames transmitted from wireless node are broadcasted and they can be received in a specific communication area. If an eavesdropper is located in the communication area, not only the receiver node but also the eavesdropper can receive the data. In this letter, we propose a distributed method for reducing the communication area by cooperative jamming among neighbor nodes. In our method, a node becomes a jamming node stochastically based on the locally available information. Through simulation evaluations, we conformed that our method is more effective than the conventional method in the situation where the node density is high. © 2019 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.