PURPOSE: To detect an appropriate clock applicable to the device of a system for detecting data on the edge of a data reproducing clock or the device of a system for detecting data using a window.

CONSTITUTION: Reading clock generation cells BR0 to BR5 for generating reading clocks and writing clock generation cells BW0 to BW3 for generating writing clocks are provided so that a 90 degree phase difference between the reading and writing clocks is provided. From even-numbered c ells BW0, BW2 and odd-numbered cells BW1 and BW3, the reproducing signals having phases different by 180 degrees are provided, sum signals between odd numbers and between even numbers are taken from these respective reproducing signals, the sum signal are further subtracted and the writing clocks are provided.