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A micro slit has been designed between micro ridges, and deformation of a biological cell passing through the micro slit has been observed in vitro. The slit, of which the width is 0.87 mm and the height is 0.010 mm, has been made between the micro ridges on the transparent polydimethylsiloxane disk and on the glass disk by the photolithography technique. The slit is placed at the middle part of a flow channel. Four kinds of cells were used in the experiment: C2C12 (mouse myoblast cells), HUVEC (human umbilical vein endothelial cells), Hepa1-6 (mouse hepatoma cells), and Neuro-2a (mouse neural crest-derived cells). The suspension of each kind of cells was introduced to the slits. The deformation of cells passing through the micro slit was observed with an inverted phase-contrast microscope. The experimental results show that cells deform to the flat circular disk and pass through the micro slit of 0.010 mm height. The deformation ratio, the passing velocity, and the shape index of cells through the slit were evaluated: Hepa1-6 is deformed with the increase of the passing velocity, and HUVEC elongates along the flow. The designed slit between micro ridges has capability to evaluate the deformability of cells.