A base station apparatus, in a mobile communication system comprised of a plurality of base station apparatuses for performing transmission using mutually different frequency bands and a mobile station apparatus capable of concurrently communicating with each of the base station apparatuses, makes a determination whether the mobile station apparatus needs to perform Inter-Frequency Measurement to communicate with the plurality of base station apparatuses, and when Inter-Frequency Measurement is needed, based on the number of frequency bands of base station apparatuses that concurrently communicate with the mobile station apparatus and the number of frequency bands that the mobile station apparatus is capable of concurrently receiving, selects one of a first Inter-Frequency Measurement method without the need for providing a measurement gap in one of the frequency bands, and a second Inter-Frequency Measurement method with the need for providing a measurement gap in one of the frequency bands.