PURPOSE: To accurately measure the level of a video signal with unknown pulse width by employing the level of a video signal for level detection which is measured one level precedently from the final one in a section wherein the level of a reference input signal exceeds a specific level value among measured levels of the video signal for level detection.

CONSTITUTION: A reference video signal 4 is compared 1 with a reference level 5 and the output signal 6 of a comparator is made effective in a section wherein the level of the signal 4 is larger than the level 5. In the section wherein the signal 6 is effective, a timing generating circuit 10 generates sampling timing 7 in each of multiples of a constant time (t) of 2 raised to (n)th power (n=0, 1...). With this timing 7, a sample holding circuit 3 measures and holds the level of a video 8 for level detection and outputs a sample holding output signal 11. Further, the signal 11 is sampled and held by a sample holding circuit 12 with the timing 7 to output a level signal 9. When the signal 6 becomes ineffective, the value of the signal 9 is the level of the video 8.