Provided are a wireless communication system, a communication method, a base station apparatus and a communication terminal wherein a further simplified procedure can be used to switch between frequency bands to be used. A wireless communication system includes a base station apparatus and a communication terminal that can communicate with the base station apparatus by use of a plurality of frequency bands. The wireless communication system comprises: a first notifying means that is disposed in one of the base station apparatus and the communication terminal and that, when having determined that there has occurred a situation to switch from one of the plurality of frequency bands, which is used as a main frequency band, to another one, notifies a switching preparation request to the other of the base station apparatus and the communication terminal; and a second notifying means that, in response to the determination that there has occurred the situation to switch from the frequency band, which is used as the main frequency band, to the other one, notifies a completion of the preparation for switching the frequency bands to the one of the base station apparatus and the communication terminal.