To specify a transmission original node of an abnormal waveform out of each node connected to a serial bus network.

A serial signal input to an oscilloscope 20A is amplified or attenuated in an input amplifier 40, and converted into quantized sampling data 47 with a sampling circuit 42. A decode/trigger circuit 44A extracts a data frame from the serial signal 41. When the data frame satisfies a prescribed trigger condition, the decode/trigger circuit 44A outputs a trigger signal 43 and a node discriminator 45 indicating the transmission original node of the data frame. A data processing circuit 46A acquires waveform data from sampling data 47 in response to the trigger signal 43, and preserves the node discriminator 45 and the waveform data to a history memory 48A by allowing them to correspond to 1:1. A display processing circuit 50 displays to superpose the waveform data transmitted from a specific node on a display unit 54 based on the node discriminator of the preserved waveform data.