A radio terminal (3) can perform carrier aggregation using a first cell (10) of a first radio station (1) and a second cell (20) of a second radio station (2). The first radio station (1) or the second radio station (2) transmits constraint information to the radio terminal (3). The constraint information contains an information element necessary to specify a reception constraint and/or transmission constraint related to the first cell (10) and/or the second cell (20) when the carrier aggregation is performed. The reception/transmission constraint is a constraint related to downlink signal reception/uplink signal transmission by the radio terminal over one or more subframe periods of the primary cell (10) and the secondary cell (20). It is thus, for example, possible to contribute to reduction in wasteful power consumption in the radio terminal in the carrier aggregation of a plurality of cells served by different radio stations.