PURPOSE: To know the continuance of a dropout and to set a reference value optionally by comparing the maximum value of pulse width with the reference value on every measurement period and deciding whether a dropout is generated or not.

CONSTITUTION: A numeral setting circuit 9 sets an optional numeral, which is compared with the reference value. For example, a control signal is supplied to a multiplexer 10, for example, on every turn on a disk and the multiplexer 10 supplies numeric data successively to a CPU2 as digital signals from a dropout generation angle detecting circuit 6, a peak holding circuit 7, a comparing circuit 8, and the numeral setting circuit 9. Dropout detection data, etc., are displayed on a display device 13 after dropout processing. Further, when a decision on a print mode is made, the detection data, etc., are printed out on a printer 14. Consequently, the continuance, i.e. width of the dropout is known and the reference value is set optionally.