To enable a medical worker to push a correct key even when the name of a slave unit displayed on a master unit does not match the key input when the slave unit is selected.

When an operation for selecting a slave unit is started, the names and identification information of available slave units are displayed as a list. When the key is input, the identification information of the corresponding slave unit is highlighted. When the key input is completed, the identification information of the corresponding slave unit is displayed in an inconspicuous manner. Thus, when the medical worker calls a patient, the name of the slave unit and the content of the key input are displayed, and therefore, the medical worker inputs the key according thereto. Also, since the identification information of the corresponding slave unit is highlighted at the time of the key input, whether the content of the key-input and the name of the salve unit match each other can easily be checked. Also, since the identification information of the corresponding slave unit becomes inconspicuous when the key input is completed, the same key is prevented from being input by mistake when another slave unit is selected.