A mobile communication system performs communication of user data directly among a plurality of radio terminals without passing through a radio base station. The communication of user data directly performed among the plurality of radio terminals is performed by using apart of radio resources assigned to the mobile communication system. The plurality of radio terminals comprise: a transmission-side terminal that transmits the user data; and a reception-side terminal that receives the user data. The reception-side terminal transmits to a radio base station in a connected state where a radio connection is established with the reception-side terminal, a transmission acknowledgment signal indicating whether the user data transmitted from the transmission-side terminal has been received.