An optical-information acquisition element comprising: a semiconductor layer of a first conductivity type; a surface-buried region of a second conductivity type buried in a part of an upper portion of the semiconductor layer so as to implement a photodiode with the semiconductor layer; a charge-accumulation region of the second conductivity type buried in a part of the upper portion of the surface-buried region, configured to accumulate a part of charges generated by the photodiode; a barrier-creating region of the first conductivity type buried adjacent to the charge-accumulation region in a part of the upper portion of the surface-buried region, wherein impurity concentrations of the surface-buried region, the barrier-creating region and the semiconductor layer are respectively selected such that a portion of the surface-buried region between the barrier-creating region and the semiconductor layer is perfectly depleted, so as to create a potential barrier in the perfectly depleted portion of the surface-buried region against an outflow of the charges accumulated in the charge accumulation region; and a charge-exhaust region of the second conductivity type buried adjacent to the surface-buried region in a limited area of the upper portion of the semiconductor layer, being contacted to the surface-buried region so as to define a path of excess charges from the charge-accumulation region via the potential barrier towards the charge-exhaust region, and being configured to store and to extract the excess charges which surmount the potential barrier and flow out from the charge-accumulation region through the path; and a wiring connected to the charge-accumulation region, wherein the wiring is configured to extract changes of potential level of the charge-accumulation region, determined by the charges accumulated in the charge-accumulation region on the basis of a height of the potential barrier in association with on and off operations of optical-communication signals, as signals. a semiconductor layer of a first conductivity type; a surface-buried region of a second conductivity type buried in a part of an upper portion of the semiconductor layer so as to implement a photodiode with the semiconductor layer; a charge-accumulation region of the second conductivity type buried in a part of the upper portion of the surface-buried region, configured to accumulate a part of charges generated by the photodiode; a barrier-creating region of the first conductivity type buried adjacent to the charge-accumulation region in a part of the upper portion of the surface-buried region, wherein impurity concentrations of the surface-buried region, the barrier-creating region and the semiconductor layer are respectively selected such that a portion of the surface-buried region between the barrier-creating region and the semiconductor layer is perfectly depleted, so as to create a potential barrier in the perfectly depleted portion of the surface-buried region against an outflow of the charges accumulated in the charge accumulation region; and a charge-exhaust region of the second conductivity type buried adjacent to the surface-buried region in a limited area of the upper portion of the semiconductor layer, being contacted to the surface-buried region so as to define a path of excess charges from the charge-accumulation region via the potential barrier towards the charge-exhaust region, and being configured to store and to extract the excess charges which surmount the potential barrier and flow out from the charge-accumulation region through the path; and a wiring connected to the charge-accumulation region, wherein the wiring is configured to extract changes of potential level of the charge-accumulation region, determined by the charges accumulated in the charge-accumulation region on the basis of a height of the potential barrier in association with on and off operations of optical-communication signals, as signals.