User equipment for transmitting an uplink control channel in multiple cells simultaneously in a radio communication system where transmission of the uplink control channel in a secondary cell is supported is disclosed. One aspect of the present invention relates to the user equipment, comprising: a transmission and reception unit configured to transmit and receive a radio channel to/from a base station via multiple cells configured for carrier aggregation; a simultaneous transmission availability determination unit configured to determine, in response to occurrence of an event where an uplink control channel is to be transmitted in the multiple cells simultaneously, whether the uplink control channel can be transmitted in the multiple cells simultaneously; and a transmission control unit configured to instruct the transmission and reception unit to transmit the uplink control channel in the multiple cells simultaneously if the uplink control channel can be transmitted in the multiple cells simultaneously and control transmission of the uplink control channel in accordance with transmission priorities corresponding to cell types if the uplink control channel cannot be transmitted in the multiple cells simultaneously.