A plurality of sensor units are sometimes mounted on an object to be detected to measure various kinds of information such as the movement, the posture, and the distortion of the object to be detected. In this case, data collected from the respective plurality of sensor units need to be synchronized with one another.
In JP-A-2004-80132, for example, for synchronous detection of communication, a master communication circuit and a plurality of slave communication circuits are prepared. When the master communication circuit communicates with one of the plurality of slave communication circuits, the master communication circuit updates count data for the start of synchronization and synchronous detection in such a manner as 0, 1, 2, . . . and transmits the count data in addition to communication data. Each of the plurality of slave communication circuits can obtain synchronization timing of communication by receiving the count data. Even if the slave communication circuit fails to receive the count data because of some reason, since the synchronization timing can be obtained according to the next update of the count data, the master communication circuit does not need to retry the transmission of the count data.