A mobile communication system comprising: a user terminal configured to perform direct device to device, D2D, communication and to perform cellular communication with a base station; and the base station configured to perform cellular communication with the user terminal, wherein the D2D communication is performed by using a part of uplink radio resources of the cellular communication, the base station is further configured to transmit, to the user terminal, a timing advance value that is used for adjustment of an uplink transmission timing in the cellular communication, and the user terminal is further configured to: receive the timing advance value from the base station, apply the timing advance value to adjustment of an uplink transmission timing in the cellular communication, apply the timing advance value to adjustment of a D2D transmission timing in the D2D communication, wherein the timing advance value which is used for the adjustment of the uplink transmission timing is utilized for the adjustment of the D2D transmission timing, and adjust a reception timing in the D2D communication to a timing, which is obtained based on the timing advance value and a predetermined offset value; characterized in that: the predetermined offset value is a fixed value which is preconfigured to the user terminal, and the user terminal is further configured to transmit information related to the timing advance value to another user terminal that performs the D2D communication with the user terminal. a user terminal configured to perform direct device to device, D2D, communication and to perform cellular communication with a base station; and the base station configured to perform cellular communication with the user terminal, wherein the D2D communication is performed by using a part of uplink radio resources of the cellular communication, the base station is further configured to transmit, to the user terminal, a timing advance value that is used for adjustment of an uplink transmission timing in the cellular communication, and the user terminal is further configured to: receive the timing advance value from the base station, apply the timing advance value to adjustment of an uplink transmission timing in the cellular communication, apply the timing advance value to adjustment of a D2D transmission timing in the D2D communication, wherein the timing advance value which is used for the adjustment of the uplink transmission timing is utilized for the adjustment of the D2D transmission timing, and adjust a reception timing in the D2D communication to a timing, which is obtained based on the timing advance value and a predetermined offset value; characterized in that: the predetermined offset value is a fixed value which is preconfigured to the user terminal, and the user terminal is further configured to transmit information related to the timing advance value to another user terminal that performs the D2D communication with the user terminal. receive the timing advance value from the base station, apply the timing advance value to adjustment of an uplink transmission timing in the cellular communication, apply the timing advance value to adjustment of a D2D transmission timing in the D2D communication, wherein the timing advance value which is used for the adjustment of the uplink transmission timing is utilized for the adjustment of the D2D transmission timing, and adjust a reception timing in the D2D communication to a timing, which is obtained based on the timing advance value and a predetermined offset value; characterized in that: the predetermined offset value is a fixed value which is preconfigured to the user terminal, and the user terminal is further configured to transmit information related to the timing advance value to another user terminal that performs the D2D communication with the user terminal. the predetermined offset value is a fixed value which is preconfigured to the user terminal, and the user terminal is further configured to transmit information related to the timing advance value to another user terminal that performs the D2D communication with the user terminal.