PURPOSE: To obtain the digital signal processing circuit with a small circuit scale and having a bit number extension function not causing rinsing.

CONSTITUTION: An analog signal is converted into an N-bit digital signal by an A/D converter 30. A signal processing circuit 31 applies prescribed processing to an output of the A/D converter 30. An N-bit time series signal 41 being an output of the signal processing circuit 31 is given to a difference detection circuit 32. The difference detection circuit 32 detects a difference between preceding and succeeding time series signals 41 to provide an output of a predetermined selection signal. A selector 33 selects an auxiliary signal 42 whose weight is smaller than N-bit to be added to the time series signal 41 based on the kind of the selection signal. An adder 35 adds the auxiliary signal 42 to the time series signal 41 to provide an (N+M)-bit time series signal 44. The time series signal 44 is converted into an analog signal by a D/A converter 38 through a signal processing circuit 37.