Disclosed are an image monitoring device capable of detecting a meaningful event on the basis of a scenario by providing a function of setting various combinations of monitoring events in a monitored image in which a plurality of monitoring events are set, and an event detecting method thereof. The data transmission system includes a central electronic apparatus and a peripheral electronic apparatus. A compare circuit coupled to the reliability oscillators compares the oscillating signals and generates a frequency compensation signal if the comparison determines that there is frequency degradation greater than a predetermined threshold.