PURPOSE: To visibly observe the status change of a high speed signal by detecting the changing point of a signal, holding the detected changing point, reading out the held contents in a prescribed period, and displaying the read result with lighting.

CONSTITUTION: A changing point detecting circuit 20 detects the changing point 31 of a signal 30 and a holding circuit 21 holds a detecting signal 32 indicating the detection. A reading part 5 reads out the contents of the circuit 21 in the prescribed period and clears the read contents in each reading. After cleared, the changing point 31 of the signal 30 is stored in the holding circuit 21 again. A display control part 11 displays the contents read out of the circuit 21 with lighting at the time of the detection signal 32, or turns off light at the time of its clear state. Since the changing point 31 of the signal 30 generated at the timing other than a reading timing is detected and displayed, the status change of a high speed signal can surely be displayed.