A web browser (5) operates a communication thread (7) and a DOM update thread (6) in parallel. When signal data including a unique identifier identical to that of signal data received is already present in a buffer region, the communication thread (7) overwrites the signal data stored in the buffer region with the signal data received. In response to reception of a signal data request event from the DOM update thread (6), the communication thread (7) transmits, to the DOM update thread (6), all the signal data stored in the buffer region. The DOM update thread (6) updates DOM for all the signal data. After the DOM update, the DOM update thread (6) transmits the signal data request event to the communication thread, and according to the updated DOM, updates display contents of display parts displayed on the web browser (5).
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