A mobile communication device, comprising: an in-vehicle radio unit configured to directly communicate with another mobile communication device, between the own and one device or between the own and two and more devices, to transmit positional information and transmission power setting information of the own mobile communication device with set transmission power, and receive information including positional information and transmission power setting information transmitted from said another mobile communication device; an in-vehicle controller configured to receive said positional information and said transmission power setting information of said another mobile communication device from said in-vehicle radio unit, record said positional information with a time and measured reception power, and keep peripheral vehicle information, in which said transmission power setting information, said time, and said measured reception power are related to one another, with said positional information; and a transmission power controller configured to receive said peripheral vehicle information from said in-vehicle controller, set transmission power based on corresponding relation between said power setting information of said another mobile communication device included in said peripheral vehicle information and said measured power in case of communication between the own and one device, set transmission power based on said positional information of said another mobile communication device in case of communication between the own and two and more devices, and output a set value of said transmission power to said in-vehicle radio unit, wherein: when a predetermined period has passed after information is included in said peripheral vehicle information, said information is deleted, and when information of said another mobile communication device is deleted from said peripheral vehicle information in communication between the own and one device, said transmission power controller sets value of transmission power to a predetermined value of maximum. an in-vehicle radio unit configured to directly communicate with another mobile communication device, between the own and one device or between the own and two and more devices, to transmit positional information and transmission power setting information of the own mobile communication device with set transmission power, and receive information including positional information and transmission power setting information transmitted from said another mobile communication device; an in-vehicle controller configured to receive said positional information and said transmission power setting information of said another mobile communication device from said in-vehicle radio unit, record said positional information with a time and measured reception power, and keep peripheral vehicle information, in which said transmission power setting information, said time, and said measured reception power are related to one another, with said positional information; and a transmission power controller configured to receive said peripheral vehicle information from said in-vehicle controller, set transmission power based on corresponding relation between said power setting information of said another mobile communication device included in said peripheral vehicle information and said measured power in case of communication between the own and one device, set transmission power based on said positional information of said another mobile communication device in case of communication between the own and two and more devices, and output a set value of said transmission power to said in-vehicle radio unit, wherein: when a predetermined period has passed after information is included in said peripheral vehicle information, said information is deleted, and when information of said another mobile communication device is deleted from said peripheral vehicle information in communication between the own and one device, said transmission power controller sets value of transmission power to a predetermined value of maximum. when a predetermined period has passed after information is included in said peripheral vehicle information, said information is deleted, and when information of said another mobile communication device is deleted from said peripheral vehicle information in communication between the own and one device, said transmission power controller sets value of transmission power to a predetermined value of maximum.