In a circuit for detecting a recorded area of an optical disk, when unnecessary noise is superimposed on the space portion of the digitized signal (RF) of an RF signal from the optical disk, a counter (20) for detecting a space portion detects the noise and resets a counter (81) for detecting a recorded area with an output signal S21 therefrom. This prevents the counter (81) for detecting a recorded area from erroneously outputting a recorded area detection signal (S12) due to the noise. As a result, a counter (80) for detecting an unrecorded area outputs an unrecorded area detection signal (S20) with high accuracy to bring a recorded area signal (RECD) from a flip-flop circuit (82) to the L level. Accordingly, an unrecorded area can be detected as it is inherently with high accuracy without being erroneously recognized as a portion in which a mark portion has been recorded.