An object of the present invention is to provide: an algae water concentration system that efficiently concentrates algae water in a culture pond into algae water containing algae having a desired size, with a simple structure and at low cost; and a method for operating the same. A cultured algae water concentration system 100 comprises: an algae water supply unit 17 that has an algae water supply container 18 that stores algae water sent from the culture pond therein, and a supply container outlet port 19 through which the algae water is taken out from the algae water supply container; and an algae water concentration unit that has a concentration container 1 for receiving and concentrating the algae water from the algae water supply unit, a filter 3 which divides the concentration container into upper and lower spaces and does not pass algae having a predetermined size or larger therethrough, a vibration device 5 that vibrates the filter in an out-of-plane direction, a concentration container algae water inlet port 7 which takes in the algae water to the concentration container and is arranged below the filter of the concentration container, a concentrated algae water outlet port 9 that is arranged below the filter of the concentration container and takes out algae water therethrough that has been concentrated in the concentration container, and a filtered water discharge port 8 that is arranged above the filter of the concentration container and discharges filtered water having passed through the filter.