A digital broadcast receiver which synchronously receives a signal having null symbol periods, said receiver comprising: frequency conversion means for generating an intermediate frequency signal by frequency-converting a reception signal in accordance with a local oscillation signal; and quadrature demodulation means for frequency-converting the intermediate frequency signal into baseband signals in accordance with a reproduced carrier signal; wherein said signal having null symbol periods is the intermediate frequency signal a low pass filter for envelope-detecting the signal having the null symbol periods and generating an envelope signal; average value computing means for sampling a plurality of digital data of the signal, and computing an average value of the plurality of digital data of the signal each time digital data are sample-inputted and generating an average value signal; and synchronism signal judging means which performs a level comparison between the average value signal and an attenuated envelope signal formed by attenuating the envelope signal with a predetermined attenuation factor, and holds the level of the attenuated envelope signal for a hold period at the output of the low pass filter from a time point at which the level of the average value signal is smaller than the level of the attenuated envelope signal and judges a certain period as a null symbol period contained in a transmission signal, said certain period lasting from the time point at which the level of the average value signal becomes smaller than the level of the attenuated envelope signal until a time point at which the level of the average value signal becomes larger than the level of the attenuated envelope signal during the hold period; frequency conversion means for generating an intermediate frequency signal by frequency-converting a reception signal in accordance with a local oscillation signal; and quadrature demodulation means for frequency-converting the intermediate frequency signal into baseband signals in accordance with a reproduced carrier signal; wherein said signal having null symbol periods is the intermediate frequency signal a low pass filter for envelope-detecting the signal having the null symbol periods and generating an envelope signal; average value computing means for sampling a plurality of digital data of the signal, and computing an average value of the plurality of digital data of the signal each time digital data are sample-inputted and generating an average value signal; and synchronism signal judging means which performs a level comparison between the average value signal and an attenuated envelope signal formed by attenuating the envelope signal with a predetermined attenuation factor, and holds the level of the attenuated envelope signal for a hold period at the output of the low pass filter from a time point at which the level of the average value signal is smaller than the level of the attenuated envelope signal and judges a certain period as a null symbol period contained in a transmission signal, said certain period lasting from the time point at which the level of the average value signal becomes smaller than the level of the attenuated envelope signal until a time point at which the level of the average value signal becomes larger than the level of the attenuated envelope signal during the hold period;