A micro-slit has been designed to measure deformation of a biological single cell passing through the slit. At the middle part of the flow channel, the slit (7 μm height, 0.4 mm width, and 0.1 mm length) has been made by photolithography technique: the combination of the micro-ridge on the transparent polydimethylsiloxane plate and the micro-groove on the glass plate. C2C12 (mouse myoblast cells) was used in the test. The flow of suspension of cells was controlled by the pressure head between the inlet and the outlet. Deformation of cells passing through the micro slit was observed with an inverted phase-contrast microscope. The experimental results show that a cell deforms to the flat circular disk and passes through the micro-slit. Although smaller cells tend to pass the micro-slit easily, some bigger cells also passed the micro-slit. Deformation ratio is independent of the size of each cell. The designed slit has capability to detect quantitative deformability of a single biological cell.