To more precisely separate a Treg cell in a closed system.

The cell-treating system 1 includes a container body 4 having a lower space 6 with a variable volume, a filter 7 arranged at a position dividing the space in the container body 4 into upper and lower parts above the lower space 6 in the container body 4, an adsorbent 8 stored in the upper space 5 above the filter 7 in the container body 4 and for adsorbing the Treg cells, a cleaning liquid-feeding opening 12 for feeding a cleaning liquid into the container body 4, a cleaning liquid-discharging opening 13 for discharging the cleaning liquid from the lower space 6, and a cell-discharging opening 14 for discharging a cell suspension containing the Treg cells from the lower space 6, wherein the filter 7 has many through-holes restricting the passage of the adsorbent 8 and allowing the passage of a cell suspension 1.