Even if no uplink transmission grant is assigned from a base station, transmit power can be controlled in uplinks with accuracy to some extent. An aspect of the present invention relates to a base station apparatus for use in a radio communication system using a shared channel, comprising: a transmit power control information generation unit configured to generate transmit power control information for a user apparatus based on an uplink signal transmitted from the user apparatus; a radio resource control unit configured to transmit the generated transmit power control information together with an uplink assignment message if an uplink radio resource is assigned to the user apparatus and to supply radio resource assignment information to transmit the generated transmit power control information as independent transmit power control information if no uplink radio resource is assigned to the user apparatus.