To solve the problem that the memory capacity of a slave unit side has to be increased if an application is downloaded.

In a cordless telephone system 1 having a master unit 10 communicatingly connected to a network 2 and the slave unit 20 connected to the master unit by radio, the master unit has a master unit side radio communicating part 11 connected to the slave unit by radio, a communication interface part 12 communicatingly connected to the network, and a master unit side controlling part 19 for downloading an application from the network and sequentially storing the downloaded data in a master unit side memory part 17, and the slave unit has a slave unit side radio communicating part 21 connected to the master unit by radio, a selecting part 31 that can optionally select the downloaded data during being stored in the master unit side memory part, and a slave unit side controlling part 30 for storing only the optionally selected downloaded data in a slave unit memory part 25 and also performing an application on the basis of the downloaded data during being stored in the slave unit memory part.