PURPOSE: To easily analyze and detect a cause to a transmission error on a transmission medium by monitoring a text and an interval of texts on the transmission medium to which a terminal equipment or the like provided with plural communication controllers and communication control functions is connected.

CONSTITUTION: A monitor communication controller 12 at the reception of a monitor start command uses a monitor initial set section 6 so as to apply initial setting in a way that a text and an interval of a text are to be monitored on a transmission medium. After the initial setting, a monitor execution section 7 monitors the text and the interval of text on the transmission medium, a monitor information processing section 8 processes the monitor information such as text interval or the like as required and sends the result to a monitor terminal equipment sequentially. The monitor terminal equipment 14 stores the received monitor information to a storage section 20 and A monitor information analysis processing section 16 analyzes a transmission error on the transmission medium based on the monitor information and displays the result of analysis by a display section 18. Thus, the pause at the occurrence of a transmission error is analyzed easily and detected.