An A/D conversion circuit for use in an information reproduction apparatus, includes an envelope generator for extracting an upper-side envelope signal and a lower-side envelope signal from a reproduced signal read out from a recording medium, and an A/D converter for converting the reproduced signal into digital data by using a reference voltage that is obtained from a difference between an upper-side reference voltage and a lower-side reference voltage. In the analog-to-digital conversion circuit, the upper-side reference voltage is set to the level of the upper-side envelope signal, while the lower-side reference voltage is set to the level of the lower-side envelope signal. With this arrangement, the difference between the level of the upper-side envelope signal and the level of the lower-side envelope signal fluctuates together with an amplitude fluctuation component and a low-frequency superimposed component in a fluctuation of the reproduced signal. The reproduced signal is converted into digital data by using the difference as a reference voltage. This makes it possible to cancel the fluctuation of the reproduced signal. Thus, this arrangement makes it possible to obtain digital data free from deviations, as well as ensuring a proper response even to a quick fluctuation in the reproduced signal.