A technique capable of realizing a power saving in a device for reproducing/recording digital signals by properly controlling a frequency of a clock. The device for reproducing/recording digital signals (device for reproducing an optical disk) includes: a difference comparing circuit for comparing a first parameter (demodulating block counter) updated each time a process for one correcting block is done in a demodulating circuit with a second parameter (error correcting block counter) updated each time a process of one correcting block is done in an error correcting circuit; and a circuit (clock controlling circuit etc.) for switching a frequency of a master clock (MCLK) depending on a comparison result of the difference comparing circuit. Thereby, the frequency of the clock can be switched both of when the demodulation for one correcting block is ended and when the correcting process for one correcting block is ended by using the switched master clock.