An on-board terminal device, comprising: a location measuring section adapted to measure a location of an own vehicle; a wireless communication section adapted to transmit own vehicle information including location information of the own vehicle to other vehicles (102, 103) and to receive other vehicle information including location information of the other vehicles (102, 103) transmitted from the other vehicles (102, 103) by execution of wireless communication with the other vehicles (102, 103); a monitoring section adapted to estimate the location of the other vehicles (102, 103), to calculate a relative distance between the own vehicle and the other vehicles (102, 103) when the wireless communication has been interrupted, and to execute an alarm process for preventing collision of the own vehicle and the other vehicles (102, 103) when the relative distance is less than a predetermined threshold; and an operator notification section adapted to provide an operator of the own vehicle with a notification on the other vehicle (102, 103), or a control signal output section adapted to output a control signal for executing travel control of the own vehicle to the own vehicle, characterized in that the monitoring section is adapted to output alarm information including information on a region where the wireless communication is difficult in the periphery of the own vehicle to the operator notification section or the control signal output section in the alarm process, the operator notification section is adapted to notify the operator of the own vehicle of the region where the wireless communication is difficult based on the alarm information, or the control signal output section is adapted to output the control signal in a case where the other vehicles (102, 103) exist in the region where the wireless communication is difficult based on the alarm information. a location measuring section adapted to measure a location of an own vehicle; a wireless communication section adapted to transmit own vehicle information including location information of the own vehicle to other vehicles (102, 103) and to receive other vehicle information including location information of the other vehicles (102, 103) transmitted from the other vehicles (102, 103) by execution of wireless communication with the other vehicles (102, 103); a monitoring section adapted to estimate the location of the other vehicles (102, 103), to calculate a relative distance between the own vehicle and the other vehicles (102, 103) when the wireless communication has been interrupted, and to execute an alarm process for preventing collision of the own vehicle and the other vehicles (102, 103) when the relative distance is less than a predetermined threshold; and an operator notification section adapted to provide an operator of the own vehicle with a notification on the other vehicle (102, 103), or a control signal output section adapted to output a control signal for executing travel control of the own vehicle to the own vehicle, characterized in that the monitoring section is adapted to output alarm information including information on a region where the wireless communication is difficult in the periphery of the own vehicle to the operator notification section or the control signal output section in the alarm process, the operator notification section is adapted to notify the operator of the own vehicle of the region where the wireless communication is difficult based on the alarm information, or the control signal output section is adapted to output the control signal in a case where the other vehicles (102, 103) exist in the region where the wireless communication is difficult based on the alarm information.