A semiconductor element comprising: a semiconductor region of a first conductivity type; a buried region of a second conductivity type, being buried in a part of an upper portion of the semiconductor region, configured to be irradiated with optical signal so as to serve as a photodiode together with the semiconductor region; an accumulation region of the second conductivity type which partially overlaps with the buried region in a part of the upper portion of the semiconductor region and is buried adjacently to the buried region, the accumulation region accumulates charges generated by the buried region and being configured to create a potential valley deeper than a potential of the bottom of a potential valley created by the buried region for majority carriers in the buried region and shallower than a depth of a potential well for the majority carriers created by a read-out region; an extraction region of the second conductivity type having higher impurity concentration than the buried region, being separated from the buried region and allocated in a part of the semiconductor region, configured to extract charges generated by the photodiode from the buried region at a particular timing as charges not to contribute to signal charges, the read-out region of the second conductivity having higher impurity concentration than the buried region, which is buried in a part of the upper portion of the semiconductor region while separated on the right side from the accumulation region and receives charges accumulated in the accumulation region; a gate insulating film formed on the semiconductor region between the buried region and the readout region; a readout gate electrode 32) formed between the accumulation region and the read-out region; an extraction-gate electrode formed on the gate insulating film between the buried region and the extraction region; characterised by an opening of a light shielding film is selectively provided such that the generation of optical charges is carried out in the semiconductor region just under the buried region implementing the photodiode; wherein the extraction-gate electrode is arranged, in the plan view, only on one side of the buried region; the gate insulating film and the extraction gate electrode on the gate insulating film implement a potential-gradient change means (31, 33) configured to control the potential of a channel formed in the upper portion of the semiconductor region between the buried region and the extraction region at the particular timing, so as to control the extraction/non-extraction of charges from the buried region to the extraction region and the transfer/non-transfer of charges to the accumulation region. a semiconductor region of a first conductivity type; a buried region of a second conductivity type, being buried in a part of an upper portion of the semiconductor region, configured to be irradiated with optical signal so as to serve as a photodiode together with the semiconductor region; an accumulation region of the second conductivity type which partially overlaps with the buried region in a part of the upper portion of the semiconductor region and is buried adjacently to the buried region, the accumulation region accumulates charges generated by the buried region and being configured to create a potential valley deeper than a potential of the bottom of a potential valley created by the buried region for majority carriers in the buried region and shallower than a depth of a potential well for the majority carriers created by a read-out region; an extraction region of the second conductivity type having higher impurity concentration than the buried region, being separated from the buried region and allocated in a part of the semiconductor region, configured to extract charges generated by the photodiode from the buried region at a particular timing as charges not to contribute to signal charges, the read-out region of the second conductivity having higher impurity concentration than the buried region, which is buried in a part of the upper portion of the semiconductor region while separated on the right side from the accumulation region and receives charges accumulated in the accumulation region; a gate insulating film formed on the semiconductor region between the buried region and the readout region; a readout gate electrode 32) formed between the accumulation region and the read-out region; an extraction-gate electrode formed on the gate insulating film between the buried region and the extraction region; characterised by an opening of a light shielding film is selectively provided such that the generation of optical charges is carried out in the semiconductor region just under the buried region implementing the photodiode; wherein the extraction-gate electrode is arranged, in the plan view, only on one side of the buried region; the gate insulating film and the extraction gate electrode on the gate insulating film implement a potential-gradient change means (31, 33) configured to control the potential of a channel formed in the upper portion of the semiconductor region between the buried region and the extraction region at the particular timing, so as to control the extraction/non-extraction of charges from the buried region to the extraction region and the transfer/non-transfer of charges to the accumulation region.