A wireless terminal device of a wireless LAN WLAN interfering system that shares at least a part of a frequency band with an interfered system, comprising: an RF part configured to perform reception processing of a wireless signal via an antenna; a first processing part for performing receiving processing in a physical layer and for receiving via said RF part, from a transmitting/receiving acccess point device of the WLAN interfering system at a point in time which is at least a maximum transmission period for one transmission by the wireless terminal prior to a start point of the sensing period, transmission prohibition period information for prohibiting transmission by the wireless terminal device until an end of the sensing period, the transmission prohibition period information comprising a Network Allocation Vector, NAV, period being a period covering the sensing period; a second processing part for performing reception processing in the media access control, MAC, layer; a sensing period information setting part for setting sensing period information that indicates a sensing period for sensing a signal which is sent from a device of the interfered system and received by the wireless terminal device using the shared frequency band; a transmission power controlling part; a transmission prohibition period information setting part for making the transmission power controlling part control the transmission power thereby to prohibit transmission processing until expiration of the NAV period; a sensing part for receiving a busy tone signal which is sent from the device of the interfered system in the sensing period and for measuring the reception strength of the received busy tone signal; and a sharing condition calculating part for using the measured reception strength of the received busy tone signal from the sensing part as a basis to determine a sharing condition, the sharing condition containing at least transmission power information as a condition, for transmission with use of the shared frequency band. an RF part configured to perform reception processing of a wireless signal via an antenna; a first processing part for performing receiving processing in a physical layer and for receiving via said RF part, from a transmitting/receiving acccess point device of the WLAN interfering system at a point in time which is at least a maximum transmission period for one transmission by the wireless terminal prior to a start point of the sensing period, transmission prohibition period information for prohibiting transmission by the wireless terminal device until an end of the sensing period, the transmission prohibition period information comprising a Network Allocation Vector, NAV, period being a period covering the sensing period; a second processing part for performing reception processing in the media access control, MAC, layer; a sensing period information setting part for setting sensing period information that indicates a sensing period for sensing a signal which is sent from a device of the interfered system and received by the wireless terminal device using the shared frequency band; a transmission power controlling part; a transmission prohibition period information setting part for making the transmission power controlling part control the transmission power thereby to prohibit transmission processing until expiration of the NAV period; a sensing part for receiving a busy tone signal which is sent from the device of the interfered system in the sensing period and for measuring the reception strength of the received busy tone signal; and a sharing condition calculating part for using the measured reception strength of the received busy tone signal from the sensing part as a basis to determine a sharing condition, the sharing condition containing at least transmission power information as a condition, for transmission with use of the shared frequency band.