A base station apparatus includes a determination unit configured to determine whether to cause a user equipment residing in a cell of the base station apparatus to conduct different frequency measurement and a transmitting unit configured to transmit a control signal to the user equipment. The user equipment includes a reception unit configured to receive the control signal from the base station apparatus of a camped cell and a cell search and quality measurement unit configured to conduct cell search at a frequency indicated by the base station apparatus and cell quality measurement. If a candidate handover target cell of the user equipment is a predefined cell, the control signal allows the user equipment to conduct the different frequency measurement in a gap period defined in a mobile communication system. If the candidate handover target cell of the user equipment is another predefined cell different from the predefined cell, the control signal allows the user equipment to conduct the different frequency measurement in the gap period defined in the mobile communication system and between discontinuous signal receptions in a discontinuous reception operating mode.