the defect detection device includes : an amplification section for amplifying a reflection signal corresponding to the intensity of light reflected from an optical disc according to a control signal indicating recording or playback ; an envelope detection section for outputting an envelope of the amplified signal ; a first pulse generation section for outputting a pulse when the level of the control signal changes ; an integration section for integrating the envelope ; a differential signal generation section for receiving the envelope as a first input signal and the integrated results as a second input signal and outputting a differential signal corresponding to the difference between these signals ; and a comparison section for comparing the differential signal with a predetermined value and outputting the results as a defect detection signal . the second input signal is changed so as to reduce the possibility that the defect detection signal may indicate the presence of a defect over the duration of the pulse .