A cell treatment device which allows cell dispersion liquid including cancer cells to pass therethrough to cause at least the cancer cells to be subjected to at least one of a physical action, a chemical action, and a biologically activating action, includes: a slit member having a first surface and a second surface and including a through hole having a slit shape penetrating from the first surface to the second surface and extending in one direction, the through hole having a tapered portion, wherein the width of a cross section of the tapered portion orthogonal in the one direction reduces from the first surface toward the second surface side, and is smaller than an average diameter of the cancer cells at an end on the second surfaces side, and the cell dispersion liquid is allowed to pass from the first surface side through the through hole to the second surface side.
