In a telephone system which includes a main device (200) and a telephone set (500) connected to the main device and which further includes a portable terminal (100) having a display unit (105), the portable terminal further has an interface portion (101, 102) for interfacing the display unit with the main device to present, as an operation screen, an image of the telephone set on a web page displayed on the display unit and to enable to use a function of the telephone set on the portable terminal by an operation on the operation screen displayed on the display unit. Preferably, the interface portion has an IP (Internet Protocol) communication network (300) for use in connecting the portable terminal to the main device. The portable terminal is, typically, a PDA (Personal Digital Assistant).