A mobile telephone switching network 3, a server 5, and a terminal equipment 7 are connected to a network 4. A portable telephone set 2 manufactured by a manufacturer 1 is connected to the mobile telephone switching network 3. Not only a service application fabricated by an application maker 6 but also a key application determining a key layout based on API published by the manufacturer 1 are uploaded to the server 5. The service and the key applications are downloaded to the portable telephone set 2 in response to a requirement from the portable telephone set 2. In the portable telephone set 2, the service application is analyzed by using the key application downloaded as mentioned above. The key layout is then determined by analyzing the service application. The key layout is displayed on the liquid crystal display of the portable telephone set 2.