The router unit is built in the main key telephone apparatus, the office line interface function and the independent power supplying function are deleted in the router unit, and the office line interface units and the power supply unit substitute for these functions. The temperature inside the cabinet, the operated state of the cooling fan, and the temperature and operating power supply voltage Vcc of the PC power supply unit, etc. are detected in the CTI personal computer, and these monitor data are transferred to the main key telephone apparatus and analyzed in the main key telephone apparatus, so as to detect abnormality in the CTI personal computer and take necessary measures against the abnormality.