The present invention provides a digital AGC device includes: a first multiplier for multiplying an in-phase signal and a filter output signal together and outputting a result of the multiplication; a second multiplier for multiplying a quadrature signal and the filter output signal together and outputting a result of the multiplication; a determination unit for making an enable signal valid and outputting the enable signal when the outputs from the first and second multipliers satisfy a condition defined according to a threshold signal; a sum-of-square averaging unit for obtaining and outputting a time average of a sum of squares of the respective outputs from the first and second multipliers when the enable signal is valid; a subtractor for obtaining and outputting a difference between a reference signal and the operation result of the sum-of-square averaging unit; and a loop filter for smoothing the output from the subtractor and outputting the smoothed output as the filter output signal. Thereby It is possible to maintain a signal average amplitude almost at a constant value.