A transmitting and receiving section transmits data to which a time measured in a first device is attached, from the first device through a communication line, and receives the data in a second device. A communication abnormality detecting section detects an abnormality in a communication between the first device and the second device on the basis of the received data. A time lag detecting section detects a time lag between the first device and the second device by comparing the time attached to the data with a time measured in the second device at the time of receiving the data.