At the time of a normal operation, a second channel change message 30b is transmitted from the AP to each terminal before a time T0 until a channel change that is included in a first channel change message 30a elapses, thereby all terminals that received this message 30b, and the AP stop changing the channel. At the time of interference occurrence, setting a time until a channel change to be filed in a third channel change message 30d to a time equal to or less than a lapse time until the channel change at the time point of transmitting this message 30d to transmit it to each terminal from the AP allows the terminal that failed to receive the message at the time of the interference occurrence to make a channel change without causing a connection to the AP to be disconnected.