A mobile base station device and billing method, which can provide an incentive for use at a specific position, while securing convenience for users, are provided. In a mobile base station device 10, upon installation by an owner, registered user terminal-specific information transmitted from a network N, and installation position information specifying the installation position, are stored. When there is transmission from a portable terminal 30, judgment is made as to whether the user of the portable terminal 30 is a registered user and as to whether transmission is from within a prescribed area; only when these conditions are satisfied, a communication connection is established at a flat rate. By this means, in the mobile base station device 10, an incentive can be provided to users for use in a specific position. Further, billing adjustments are made for use at other than the specific position, but communication service is not completely forbidden, and user convenience is secured.